Protocol Specification

Learn about how the atProtocol works

Introduction

The atProtocol is the underlying protocol that the atPlatform implements. It is a simple internet protocol for interacting with the Root Server and Secondary Server. The atPlatform builds on top of the atProtocol by taking full advantage of the features of the atProtocol.

See our Protocol Specification on our GitHub page under our at_protocol repository.

If you see a typo, mistake, or want to suggest a feature, please see our contributing guidelines. All contributions are welcome.

Root Server

A Root Server should provide a lookup of where a Secondary Server for an atSign is running. This is similar to a DNS server.

When asking a Root Server for the lookup of a particular atSign the Root Server should respond with a null if the name does not exist and if the name exists the DNS name or address of the atServer and the IP port number for that atSign should be returned.

Response:

<host>:<port>

The Root Server only has one verb - @exit and all other inputs are considered to be lookup requests.

Secondary Server

A Secondary Server is where an atSign user’s personal data should be stored. One interacts with a secondary using the verbs exposed by the protocol.

A Secondary Server should have 4 major sub components:

  1. Key Store
  2. Commit Log
  3. Access Log
  4. Notification Log

Verbs described in the document should be used to create, update, delete and retrieve information from the above sub components.

1. Key Store

Key store is a place where user data in a Secondary Server should be saved as key and value pairs. Apart from the value, an atSign user should be able to add certain metadata for a key.

Key

A key in the atProtocol can be formed by using any alphanumeric and special characters (UTF-8) excluding “@”, “:” and a white space (" “). A key in a secondary can be any of the following 5 types:

  1. Public Key
  • A public key is a key which can be looked up by any atSign user.
  • A public key should be part of the scan verb result.
  • Format of the public key should be public::<@sign>.

Example:

public:location@alice

The owner of the secondary should be allowed to update or delete the value of a public key.

  1. Private Key
  • A private key is a key which cannot be looked up by any atSign user other than the one who created it.
  • A private key should not be returned in a scan verb result.
  • Format of the private key should be privatekey::<@sign>.

Example:

privatekey:pk1@alice

The owner of the secondary should be allowed to update or delete the value of a private key.

  1. User key
  • A user key can only be looked up by an atSign user with whom the data has been shared.
  • A user key should be part of the scan verb result only for the user who created it and the specific user it has been shared with.
  • Format of the key shared with someone else should be <Shared with atSign>::<Created by atSign>.

Example:

@bob:phone@alice

Note: Above Key should be part of scan verb result only for @alice and @bob

The owner of the secondary should be allowed to update or delete the value of a user key.

  1. Internal Key
  • Internal keys start with an underscore(_) and are not displayed in scan results. Internal keys can be looked up only by the owner of the secondary
  1. Cached Key
  • A cached key is a key that was originally created by another atSign user but is now cached on the Secondary Server of another user’s atSign as they were given permission to cache it.
  • A cached key should be listed in the scan verb result for the atSign user who cached it.
  • Format of the key shared with someone else should be cached:<Shared with atSign>::<Created by atSign>.

Example:

cached:@bob:phone@alice

The user who has cached the key should not be allowed to update the cached key.

An atSign user who has created and shared the key should be allowed to update a cached key, and if the “autoNotify” config parameter is set to true, the updated value should be notified (please refer to the notify verb) and the cached key updated with the new value.

If the user who originally shared the keys set the CCD (Cascade delete) to true, the cached key will be deleted when the original key is deleted.

Value

Text or binary values can be saved in a Secondary Server. The size of the valuue saved in a secondary is bound by the config parameter “maxBufferSize”.

A user should be made aware of this limitation by using the stats verb.

If a binary value is being saved on a Secondary Server, the “isBinary” attribute on the metadata should be set to true by the convention.

  1. Reference Value

A Secondary Server should support referencing another key’s value.

A reference value should be in the format “atsign://”.

For example, ‘phone@bob(key)’ is 1234 (value). Now another key called altPhone@bob can refer to phone@bob by referencing it as altPhone@bob == atsign://phone@bob.

When the user does a lookup on the key that contains a reference, the Secondary Server should return a fully resolved value.

  1. Metadata

Metadata of a key should describe the following properties of the value being inserted.

Meta AttributeAuto create?Description
createdOnYesDate and time when the key has been created.
createdByYesatSign that has created the key
updatedOnYesDate and tiem when the key has been last updated
sharedWithNoatSign of the user with whom the key has been shared. Can be null if not shared with anyone.
ttlNoTime to live in milliseconds.
expiresOnYesA Date and Time derived from the ttl (now + ttl). A Key should be auto deleted once it expires.
ttbNoTime to birth in milliseconds.
availableFromYesA Date and Time derived from the ttl (now + ttl). A Key should be only available after availableFrom
isCachedNoTrue if the key is cached
ttrNoTime in milliseconds after which the cached key needs to be refreshed. Ttr of -1 indicates that the key can be cached forever.
refreshAtNoA Date and Time derived from the ttr. The time at which the key gets refreshed.
ccdNoIndicates if a cached key needs to be deleted when the atSign user who has originally shared it deletes it.
isBinaryNoTrue if the value is a binary value.
isEncryptedNoTrue if the value is encrypted.

2. Commit Log

A Secondary Server should record any create, update and delete operations in a commit log. The Commit Log should record these operations with a unique commit id so that users of the secondary can lookup operations that happened on or after a given commit id.

A Secondary Server should provide a way to compact the Commit Log based on time and size.

3. Access Log

A Secondary Server should record the following user actions: user login, user authentication and lookup. The Access Log should record these operations so that users of the secondary can retrieve various statistics such as the most visited atSign or most visited keys.

A Secondary Server should provide a way to compact the Access Log based on time and size.

4. Notification Log

A Secondary Server should record any notifications that have been received and sent. Please check the notify verb specification for details on how a notification should be sent.

A Secondary Server should provide a way to compact the Notification Log based on time and size.

Standard Keys

A Secondary Server should have the following standard keys:

KeyDescription
public:publickey@Public key used by other atSigns for encryption
public:signing_publickey@Public key used on a pol handler to a verify a signed challenge
@signing_privatekey@Private key used to sign a challenge on a pol request
:shared_key@Symmetric key used to encrypt/decrypt self atSign data

Configuration Parameters

A Secondary Server should honor the following configuration parameters.

KeyValid ValuesDescription
autoNotifytrue/falseIf set to true, a Secondary Server should automatically notify another atSign user when a key has been shared with them. Please refer to the notify verb spec for details.
bufferLimitNumber of bytesMaximum size of a value for a key that can be transferred to a Secondary Server
inbound_max_limitAn IntegerMaximum number of inbound connections that a Secondary Server can accept
outbound_max_limitAn IntegerMaximum number of outbound connections that a secondary can make to another Secondary Server
inbound_idle_time_millisTime in millisecondsMaximum time the inbound connection can active
outbound_idle_time_millisTime in millisecondsMaximum time the outbound connection can be active.

Block List

A user of the Secondary Server should be able to decide who is allowed to connect to a Secondary Server. The config verb should be used to configure this. Once added, a Secondary Server should honor the list at the time of accepting new connections from an atSign user using the from verb.

Verbs

The from verb

Synopsis:

The from verb is used to tell a secondary whom you claim to be.

Following regex represents the syntax of the from verb:

r'^from:(?<@sign>@?[^@\s]+$)'

Example:

from:@bob

Response:

If the user who is trying to connect is the owner of the Secondary Server, then the from verb should respond with the following response.

key:<sessionId@sign:uuid>

If the user who is trying to connect is not the owner of the Secondary Server, then the from verb should respond with the following response.

proof:<sessionId>@<@sign>:<UUID>

If the user is not allowed to connect to the Secondary Server, then it should respond back with the following error and close the connection to the server.

error:AT0013-Connection Exception

Description:

The from verb is used to tell the Secondary Server what atSign you claim to be. With the from verb, one can connect to one’s own Secondary Server or someone else’s Secondary Server. In both cases, the Secondary Server responds back with a challenge to prove that you are who you claim to be. This is part of the authentication mechanism of the atProtocol.

This authentication mechanism varies based on whether you are connecting to your own secondary (cram) or someone else’s secondary (pol).

Options:

<atSign> Required: Yes. atSign with which you are connecting to a Secondary Server

The cram verb

Synopsis:

The cram verb is used to bootstrap authenticate one’s own self as the owner of a Secondary Server. It is intended to be used once until a set of PKI keys are cut on the owner’s mobile device and from then on we use the pkam verb.

The following regex represents the syntax of the cram verb:

r'^cram:(?<digest>.+$)'

Example:

cram:<digest>

Response:

If the user gets the challenge right, the prompt should change to the atSign of the user.

<@sign>@

If the user gets the cram authentication wrong, then it should respond back with the following error and close the connection to the server.

error:AT0401-Client authentication failed

Description:

The cram verb follows the from verb. As an owner of the Secondary Server, you should be able to take the challenge thrown by the from verb and encrypt using the shared key that the server has been bound with. Upon receiving the cram verb along with the digest, the server decrypts the digest using the shared key and matches it with the challenge. If they are the same, then the secondary lets you connect to the Secondary Server and changes the prompt to your atSign.

Options:

<digest> Required: Yes. Description: Encrypted challenge

The pol verb

Synopsis:

The pol verb is part of the pkam process to authenticate oneself while connecting to someone else’s Secondary Server. The term ‘pol’ means ‘proof of life’ as it provides a near realtime assurance that the requestor is who it claims to be.

Following regex represents the syntax of the pol verb:

r'^pol$'

Example:

pol

Response::

If the user gets the challenge right the prompt should change to the atSign of the user.

<@sign>@

If the user gets the cram authentication wrong, then it should respond back with the following error and close the connection to the server.

error:AT0401-Client authentication failed

Description:

The pol verb follows the from verb. ‘pol’ indicates another secondary that the user who is trying to connect is ready to authenticate himself. For example, if @bob is trying to connect to @alice, @bob would take the key and value from the proof response of the verb and create a public key and value which then can be looked up by @alice. After @alice looks up @bob’s secondary @alice’s secondary should change the prompt to @bob.

Options:

NA

The scan verb

Synopsis:

The scan verb is used to see the keys in an atSign’s secondary server. Following regex represents the syntax of the scan verb: r'^scan$|scan(:showhidden:(?<showhidden>true|false))?(:(?<forAtSign>@[^:@\s]+))?(:page:(?<page>\d+))?( (?<regex>\S+))?$'

Example:

scan

scan:showhidden:true

scan: <regex>

Response:

The Secondary Server should return the keys within the secondary server if the scan verb executed successfully. The Secondary Server will respond accordingly to whether the atSign is authenticated or not. data:[<keys>]

The update verb

Synopsis:

The update verb is used to insert key/value pairs into a Key Store. An update can only be run by the owner of a Secondary Server on his/her own Secondary Server.

Following regex represents the syntax of the update verb:

r'^update:(?:ttl:(?<ttl>\d+):)?(?:ttb:(?<ttb>\d+):)?(?:ttr:(?<ttr>(-?)\d+):)?(ccd:(?<ccd>true|false):)?((?:public:)|(@(?<for@sign>[^@:\s]-):))?(?<atKey>[^:@]((?!:{2})[^@])+)(?:@(?<@sign>[^@\s]-))? (?<value>.+$)'

Example:

update:location@bob bob's location value

update:ttl:600000:location@bob bob's location value but key expires in 10 minutes

update:@alice:phone@bob bob's phone number shared to @alice

Response:

The Secondary Server should return the commit id from Commit Log if the update is successful.

data:<CommitId>

If the user provides the invalid update command, then it should respond with the following error and close the connection to the server

error:AT0003-Invalid Syntax

Description:

The update verb should be used to perform create/update operations on the Secondary Server. The update verb requires the owner of the secondary to authenticate himself/herself to the Secondary Server using from and cram verbs.

If a key has been created for another @sign user, the Secondary Server should honor “autoNotify” configuration parameter.

Options:

<ttl> Required: No. Description: Time to live in milliseconds.

<ttb> Required: No. Description: Time to birth in milliseconds.

<ttr> Required: No. Description: Time to refresh in milliseconds.

-1 is a valid value which indicates that the user with whom the key has been shared can keep it forever and the value for this key won’t change forever.

<ccd> Required: No. Description: A value of “true” indicates that the cached key needs to be deleted when the atSign user who has originally shared it deletes it.

<for@sign> Required: Yes, (Not required when the key is a public key). Description: atSign of the user with whom the key has been shared

<@sign> Required: Yes. Description: atSign of the owner.

<value> Required: Yes. DEscription: Value for the key.

The update:meta verb

Synopsis:

The update:meta verb should be used to update the metadata of a key atSign user without having to send or save the value again.

Following is the regex for the update:meta verb

^update:meta:((?:public:)|((?<forAtSign>@?[^@\s]-):))?(?<atKey>((?!:{2})[^@])+)@(?<atSign>[^@:\s]-)(:ttl:(?<ttl>\d+))?(:ttb:(?<ttb>\d+))?(:ttr:(?<ttr>\d+))?(:ccd:(?<ccd>true|false))?(:isBinary:(?<isBinary>true|false))?(:isEncrypted:(?<isEncrypted>true|false))?$

Example:

update:meta:phone@bob:isBinary:true

update:meta:@alice:phone@bob:ttl:600000:isBinary:true:isEncrypted:true

Response:

The Secondary Server should return the commit id from Commit Log if the update is successful.

data:<CommitId>

If the user provides the invalid update meta command, then it should respond with the following error and close the connection to the server.

error:AT0003-Invalid Syntax

Description:

The update:meta verb should be used to perform create/update operations on the Secondary Server. The update:meta verb requires the owner of the secondary to authenticate himself/herself to the Secondary Server using from and cram verbs.

The Secondary Server should allow creation of keys with null values. If a key has been created for another @sign user, the Secondary Server should honor “autoNotify” configuration parameter.

Options::

<ttl> Required: No. Description: Time to live in milliseconds

<ttb> Required: No. Description: Time to birth in milliseconds

<ttr> Required: No. Description: Time to refresh in milliseconds

-1 is a valid value which indicates that the user with whom the key has been shared can keep it forever and the value for this key won’t change forever

<ccd> Required: No. Description: A value of “true” indicates that the cached key needs to be deleted when the atSign user who has originally shared it, deletes it.

<for@sign> Required: Yes (Not required when the key is a public key). Description: atSign of the user with whom the key has been shared

<@sign> Required: Yes. Description: atSign of the owner

The lookup verb

Synopsis:

The lookup verb should be used to lookup the value shared by another atSign user.

The following is the regex of the lookup verb:

lookup:((?<operation>meta|all):)?(?<atKey>(?:[^:]).+)@(?<@sign>[^@\s]+)$

Example:

lookup:phone@alice

lookup:meta:phone@alice

lookup:all:phone@alice

Response:

If the operation is not specified the Secondary Server should just respond back with the value saved by the user as is.

data:<value>

If the operation is to lookup the metadata only then the result should be wrapped in a JSON in the following format:

data:<Metadata in a JSON>

Example:

data: 
{
    "createdBy":"@bob",
    "updatedBy":"@bob",
    "createdAt":"2020-10-21 09:46:48.982Z",
    "updatedAt":"2020-10-21 09:46:48.982Z",
    "availableAt":"null",
    "expiresAt":"null",
    "refreshAt":"2020-10-21 09:46:58.982Z",
    "status":"active",
    "version":0,
    "ttl":null,
    "ttb":null,
    "ttr":10000,
    "ccd":false,
    "isBinary":false,
    "isEncrypted":false
 }

If the operation is to lookup the metadata and the data together, then the result should be wrapped in a JSON in the following format:

data:<Value and Metadata in a JSON>

Example:

data:
{
	"key":"@alice:country@bob",
	"data":"USA",
	"metaData":
	{
	 "createdBy":"@bob",
       "updatedBy":"@bob",
       "createdAt":"2020-10-21 09:46:48.982Z",
       "updatedAt":"2020-10-21 09:46:48.982Z",
       "availableAt":"null",
       "expiresAt":"null",
       "refreshAt":"2020-10-21 09:46:58.982Z",
       "status":"active",
       "version":0,
       "ttl":null,
       "ttb":null,
       "ttr":10000,
       "ccd":false,
       "isBinary":false,
       "isEncrypted":false
     }
}

If the other Secondary Server on which the lookup needs to be performed is down then the secondary should return the following error and keep the connection alive.

error:AT0007-Secondary Server not found.

If the lookup command is not valid, then the Secondary Server should return the following error and close the connection:

error:AT0003-Invalid Syntax

For whatever reasons, If the handshake with another secondary fails, then the Secondary Server should return the following error:

data:AT0008-Handshake failure

Description:

The lookup verb should be used to fetch the value of the key shared by another atSign user. If there is a public and user key with the same name then the result should be based on whether the user is trying to lookup is authenticated or not. If the user is authenticated then the user key has to be returned, otherwise the public key has to be returned.

The plookup verb

Synopsis:

The plookup verb enables to lookup the value of the public key shared by another atSign user.

Following is the regex of the plookup verb:

^plookup:((?<operation>meta|all):)?(?<atKey>[^@\s]+)@(?<@sign>[^@\s]+)$

Example:

plookup:publickey@alice

plookup:meta:publickey@alice

plookup:all:publickey@alice

Response:

The Secondary Server should return the value or metadata or the value and metadata together based on the option specified.

The response structure should be exactly the same as the lookup verb.

If the other Secondary Server on which the lookup needs to be performed is not available, then the secondary should return the following error and keep the connection alive.

error:AT0007-Secondary Server not found.

If the lookup command is not valid, then the Secondary Server should return the following error and close the connection:

error:AT0003-Invalid Syntax

The llookup verb

Synopsis:

The llookup verb should be used to look up one’s own secondary and this should return the value as is (i.e. without any resolution).

The following is the regex of the llookup verb:

^llookup:((?<operation>meta|all):)?(?:cached:)?((?:public:)|(@(?<for@sign>[^@:\s]-):))?(?<atKey>[^:]((?!:{2})[^@])+)@(?<@sign>[^@\s]+)$

Example:

llookup:public:publickey@bob

llookup:all:phone@bob

Response:

The Secondary Server should return the value or metadata or the value and metadata together based on the option specified.

The response structure should be exactly the same as the lookup verb.

If the other Secondary Server on which the lookup needs to be performed is down then the secondary should return the following error and keep the connection alive.

error:AT0007-Secondary Server not found.

If the lookup command is not valid, then the Secondary Server should return the following error and close the connection:

error:AT0003-Invalid Syntax

Description:

The llookup verb should be used to fetch the value of the key in the owner’s secondary store as is without resolving it. For example if a key contains a reference as a value, the lookup verb should resolve it to a value whereas llookup should return the value as is.

Example:

If phone@bob is “1234” and altphone@bob is “atsign://phone@bob”, then lookup of altphone@bob should return “1234” where as llookup of altphone@bob should return “atsign://phone@bob”.

The pkam verb

Synopsis:

The pkam verb is used to authenticate one’s own self as an owner of a Secondary Server using a PKI style authentication.

Following regex represents the syntax of the pkam verb:

^pkam:(?<signature>.+$)

Example:

pkam:<digest>

Response:

If the user gets the challenge right, the prompt should change to the atSign of the user.

<@sign>@

If the user gets the pkam authentication wrong, then it should respond back with the following error and close the connection to the server.

error:AT0401-Client authentication failed

Description:

The pkam verb follows the from verb. As an owner of the Secondary Server, you should be able to take the challenge thrown by the from verb and encrypt using the private key of the RSA key pair with what the server has been bound with. Upon receiving the cram verb along with the digest, the server decrypts the digest using the public key and matches it with the challenge. If they are the same then the secondary lets you connect to the Secondary Server and changes the prompt to your atSign.

Options:

<digest> Required: Yes. Description: Encrypted challenge

The stats verb

Synopsis:

The stats verb should be used to get the statistics of an atSign.

Following is the regex of the stats verb

stats(?<statId>:((?!0)\d+)?(,(\d+))-)?

Response:

if the user gives stats all the statistics will be returned as JSON. Following statistics are provided:

  1. activeInboundConnections
  2. activeOutboundConnections
  3. lastCommitId
  4. secondaryStorageSize
  5. topAtSigns
  6. topKeys

Example:

data: [{"id":"1","name":"activeInboundConnections","value":"1"}, {"id":"2","name":"activeOutboundConnections","value":"0"}, {"id":"3","name":"lastCommitID","value":"1"}, {"id":"4","name":"secondaryStorageSize","value":12560}, {"id":"5","name":"topAtSigns","value":{"@bob":1}}, {"id":"6","name":"topKeys","value":{"publickey@alice":1}}]

Individual statistics can be retrieved using the respective Id.

@alice@stats:1
data: [{"id":"1","name":"activeInboundConnections","value":"1"}]

The sync verb

The sync verb enables to synchronize the keys between the local Secondary Server and remote Secondary Server.

Following is the regex:

sync:(?<from_commit_seq>[0-9]+$|-1)

Response:

The sync verb returns a json array of the commit entries from the given commit id to the current commit id. Further, the sync verb accepts -1 as argument which returns all the commit entries.

data:[{"atKey":"@bob:phone@alice","operation":"+","opTime":"2020-10-26 11:57:43.732","commitId":0,"value":"12345","metadata":{"ttr":"36000000","ccd":"false"}},
{"atKey":"@bob:shared_key@alice","operation":"-","opTime":"2020-10-26 09:44:54.382219Z","commitId":1}]

The notify verb

Synopsis:

The notify verb enables us to notify the atSign user of some data event.

The following is the regex for the notify verb

notify:((?<operation>update|delete):)?(ttl:(?<ttl>\d+):)?(ttb:(?<ttb>\d+):)?(ttr:(?<ttr>(-)?\d+):)?(ccd:(?<ccd>true|false):)?(@(?<forAtSign>[^@:\s]-)):(?<atKey>[^:]((?!:{2})[^@])+)@(?<atSign>[^@:\s]+)(:(?<value>.+))?

Example:

notify:update:@alice:test@bob

notify:delete:@alice:test@bob

Response:

When a key is notified successfully, returns

data:<notificationId>

where the notificationId is the id of the sent notification.

Description:

When an atSign user notifies the key to another atSign user, an entry has to be created in received notifications list on the user who has shared the key and an entry has to be created in sent notifications list on the user to whom the key is to be notified. When auto notify is set to true, when a key is created/updated and deleted notification is triggered to another atSign user.

Notify List

Synopsis:

Notify list returns a list of notifications.

The following is the regex

notify:(list (?<regex>.-)|list$)

Example:

notify:list

Response:

If the user is the owner, returns a list of received notifications. If a user is pol authenticated user, returns a list of sent notifications

data:[{"id":"0e5e9e89-c9cb-423b-8972-8c5487215990","from":"@alice","to":"@bob","key":"@bob:phone@alice","value":12345,"operation":"update","epochMillis":1603714122636}]

Notify Remove

Synopsis:

Notify remove removes a notification from the notification log.

The following is the regex

notify:(remove:(?<notificationId>[^:]+$))

Example:

notify:remove:0e5e9e89-c9cb-423b-8972-8c5487215990

Response:

If successful, returns

data:success

The monitor verb

Synopsis:

The monitor verb streams received notifications.

The following is the regex

^monitor$|^monitor ?(?<regex>.-)?)$

Example:

monitor

Response:

Returns a stream of notifications

@alice@monitor
notification: {"id":"773e226d-dac2-4269-b1ee-64d7ce93a42f","from":"@bob","to":"@alice","key":"@alice:phone@bob","value":null,"operation":"update","epochMillis":1603714720965}

Description:

The monitor verb accepts an optional parameter to filter the notifications by passing filter criteria as regex to monitor verb.

Error Codes

Error CodeError MessageDescription
AT0001Server exceptionException occurs when there is an issue while starting the server.
AT0002Datastore exceptionException occurs during keystore operations (GET/PUT/DELETE).
AT0003Invalid syntaxException occurs if we give any invalid command to the server.
AT0004Socket errorException occurs when socket connection to secondary cannot be established.
AT0005Buffer limit exceededThis exception occurs when input/output message size reaches the maximum limit configured in the server.
AT0008Handshake failurethis exception is for any exception during the handshake process of two secondaries.
AT0009Unauthorized client in the requestOccurs when an unsuccessful handshake happens between two secondaries.
AT0010Internal server errorThis is for any server related errors.
AT0011Internal server exceptionThis exception is used for any server related exceptions.
AT0012Inbound connection limit exceededThis exception will occur when the number of active clients reaches the maximum limit configured.
AT0401Client authentication failedThis exception occurs when client authentication fails or client tries to execute any verb which needs authentication before successful authentication.
AT0013Connection exceptionThis will occur when a blocked user tries to connect to the secondary.
AT0014Unknown AtClient exceptionThis exception will be thrown while performing any operations (GET/UPDATE/DELETE) using AtClient SDK.
AT0015Key not foundThis exception will be thrown when the key is not available for encryption/decryption.
AT0021Unable to connect to secondaryThis exception will occur when we are unable to connect to secondary.