MQTT broker state store protocol

The MQ state store is a distributed storage system within the Azure IoT Operations cluster. The state store offers the same high availability guarantees as MQTT messages in MQTT broker. According to the MQTT5/RPC protocol guidelines, clients should use MQTT5 to interact with the MQ state store. This article provides protocol guidance for developers who need to implement their own MQTT broker state store clients.

State store protocol overview

The MQ state store supports the following commands:

• SET <keyName> <keyValue> <setOptions>
• GET <keyName>
• DEL <keyName>
• VDEL <keyName> <keyValue> ## Deletes a given <keyName> if and only if its value is <keyValue>

The protocol uses the following request-response model:

• Request. Clients publish a request to a well-defined state store system topic. To publish the request, clients use the required properties and payload described in the following sections.
• Response. The state store asynchronously processes the request and responds on the response topic that the client initially provided.

The following diagram shows the basic view of the request and response:

State store system topic, QoS, and required MQTT5 properties

To communicate with the state store, clients must meet the following requirements:

• Use MQTT5. For more information, see the MQTT 5 specification.
• Use QoS 1 (Quality of Service level 1). QoS 1 is described in the MQTT 5 specification.
• Have a clock that is within one minute of the MQTT broker's clock.

To communicate with the state store, clients must PUBLISH requests to the system topic statestore/v1/FA9AE35F-2F64-47CD-9BFF-08E2B32A0FE8/command/invoke. Because the state store is part of Azure IoT Operations, it does an implicit SUBSCRIBE to this topic on startup.

To build a request, the following MQTT5 properties are required. If these properties aren't present or the request isn't of type QoS 1, the request fails.

• Response Topic. The state store responds to the initial request using this value. As a best practice, format the response topic as clients/{clientId}/services/statestore/_any_/command/invoke/response. Setting the response topic as statestore/v1/FA9AE35F-2F64-47CD-9BFF-08E2B32A0FE8/command/invoke or as one that begins with clients/statestore/v1/FA9AE35F-2F64-47CD-9BFF-08E2B32A0FE8 is not permitted on a state store request. The state store disconnects MQTT clients that use an invalid response topic.
• Correlation Data. When the state store sends a response, it includes the correlation data of the initial request.

The following diagram shows an expanded view of the request and response:

Supported commands

The commands SET, GET, and DEL behave as expected.

The values that the SET command sets, and the GET command retrieves, are arbitrary binary data. The size of the values is only limited by the maximum MQTT payload size, and resource limitations of MQ and the client.

SET options

The SET command provides more optional flags beyond the basic keyValue and keyName:

• NX. Allows the key to be set only if it doesn't exist already.
• NEX <value>. Allows the key to be set only if the key doesn't exist or if the key's value is already set to <value>. The NEX flag is typically used for a client renewing the expiration (PX) on a key.
• PX. How long the key should persist before it expires, in milliseconds.

VDEL options

The VDEL command is a special case of the DEL command. DEL unconditionally deletes the given keyName. VDEL requires another argument called keyValue. VDEL only deletes the given keyName if it has the same keyValue.

Payload format

The state store PUBLISH payload format is inspired by RESP3, which is the underlying protocol that Redis uses. RESP3 encodes both the verb, such as SET or GET, and the parameters such as keyName and keyValue.

Case sensitivity

The client must send both the verbs and the options in upper case.

Request format

Requests are formatted as in the following example. Following RESP3, the * represents the number of items in an array. The $ character is the number of characters in the following line, excluding the trailing CRLF.

The supported commands in RESP3 format are GET, SET, DEL, and VDEL.

*{NUMBER-OF-ARGUMENTS}<CR><LF>
${LENGTH-OF-NEXT-LINE}<CR><LF>
{COMMAND-NAME}<CR><LF>
${LENGTH-OF-NEXT-LINE}<CR><LF> // This is always the keyName with the current supported verbs.
{KEY-NAME}<CR><LF>
// Next lines included only if command has additional arguments
${LENGTH-OF-NEXT-LINE}<CR><LF> // This is always the keyValue for set
{KEY-VALUE}<CR><LF>

The following example output shows state store RESP3 payloads:

*3<CR><LF>$3<CR><LF>set<CR><LF>$7<CR><LF>SETKEY2<CR><LF>$6<CR><LF>VALUE5<CR><LF>
*2<CR><LF>$3<CR><LF>get<CR><LF>$7<CR><LF>SETKEY2<CR><LF>
*2<CR><LF>$3<CR><LF>del<CR><LF>$7<CR><LF>SETKEY2<CR><LF>
*3<CR><LF>$4<CR><LF>vdel<CR><LF>$7<CR><LF>SETKEY2<CR><LF>$3<CR><LF>ABC<CR><LF>

Response format

When the state store detects an invalid RESP3 payload, it still returns a response to the requestor's Response Topic. Examples of invalid payloads include an invalid command, an illegal RESP3, or integer overflow. An invalid payload starts with the string -ERR and contains more details.

If a client sends an invalid payload, the state store sends a payload like the following example:

-ERR syntax error

SET response

When a SET request succeeds, the state store returns the following payload:

+OK<CR><LF>

If a SET request fails because a condition check as specified in the NX or NEX set options that means the key cannot be set, the state store returns the following payload:

-1<CR><LF>

GET response

When a GET request is made on a nonexistent key, the state store returns the following payload:

$-1<CR><LF>

When the key is found, the state store returns the value in the following format:

${NumberOfBytes}<CR><LF>
{KEY-VALUE}

The output of the state store returning the value 1234 looks like the following example:

$4<CR><LF>1234<CR><LF>

DEL and VDEL response

The state store returns the number of values it deletes on a delete request. Currently, the state store can only delete one value at a time.

:{NumberOfDeletes}<CR><LF> // Will be 1 on successful delete or 0 if the keyName is not present

The following output is an example of a successful DEL command:

:1<CR><LF>

If a VDEL request fails because the value specified does not match the value associated with the key, the state store returns the following payload:

-1<CR><LF>

-ERR responses

The following is the current list of error strings. Your client application should handle unknown error strings to support updates to the state store.

Error string returned from state store	Explanation
the request timestamp is too far in the future; ensure that the client and broker system clocks are synchronized	Unexpected request timestamp caused by the state store and client clocks are not in sync.
a fencing token is required for this request	Error occurs if a key is marked with a fencing token, but the client doesn't specify the fencing token.
the request fencing token timestamp is too far in the future; ensure that the client and broker system clocks are synchronized	Unexpected fencing token timestamp caused by the state store and client clocks are not in sync.
the request fencing token is a lower version that the fencing token protecting the resource	Incorrect request fencing token version. For more information, see [Versioning and hybrid logical clocks].(#versioning-and-hybrid-logical-clocks)
the quota has been exceeded	The state store has a quota of how many keys it can store, which is based on the memory profile of the MQTT broker that's specified.
syntax error	The payload sent doesn't conform to state store's definition.
not authorized	Authorization error
unknown command	Command isn't recognized.
wrong number of arguments	Incorrect number of expected arguments.
missing timestamp	When clients do a SET, they must set the MQTT5 user property __ts as an HLC representing its timestamp.
malformed timestamp	The timestamp in the __ts or the fencing token isn't legal.
the key length is zero	Keys can't be zero length in state store.

Versioning and hybrid logical clocks

This section describes how the state store handles versioning.

Versions as Hybrid Logical Clocks

The state store maintains a version for each value it stores. The state store could use a monotonically increasing counter to maintain versions. Instead, the state store uses a Hybrid Logical Clock (HLC) to represent versions. For more information, see the articles on the original design of HLCs and the intent behind HLCs.

The state store uses the following format to define HLCs:

{wallClock}:{counter}:{node-Id}

The wallClock is the number of milliseconds since the Unix epoch. counter and node-Id work as HLCs in general.

When clients do a SET, they must set the MQTT5 user property __ts as an HLC representing its timestamp, based on the client's current clock. The state store returns the version of the value in its response message. The response is also specified as an HLC and also uses the __ts MQTT5 user property. The returned HLC is always greater than the HLC of the initial request.

Example of setting and retrieving a value's version

This section shows an example of setting and getting the version for a value.

A client sets keyName=value. The client clock is October 3, 11:07:05PM GMT. The clock value is 1696374425000 milliseconds since Unix epoch. Assume that the state store's system clock is identical to the client system clock. The client does the SET command as described previously.

The following diagram illustrates the SET command:

The __ts (timestamp) property on the initial set contains 1696374425000 as the client wall clock, the counter as 0, and its node-Id as CLIENT. On the response, the __ts property that the state store returns contains the wallClock, the counter incremented by one, and the node-Id as StateStore. The state store could return a higher wallClock value if its clock were ahead, based on the way HLC updates work.

This version is also returned on successful GET, DEL, and VDEL requests. On these requests, the client doesn't specify a __ts.

The following diagram illustrates the GET command:

If a given key is later updated with a new SET, the process is similar. The client should set its request __ts based on its current clock. The state store updates the value's version and returns the __ts, following the HLC update rules.

Clock skew

The state store rejects a __ts (and also a __ft) that is more than a minute ahead of the state store's local clock.

The state store accepts a __ts that is behind the state store local clock. As specified in the HLC algorithm, the state store sets the version of the key to its local clock because it's greater.

Locking and fencing tokens

This section describes the purpose and usage of locking and fencing tokens.

Background

Suppose there are two or more MQTT clients using the state store. Both clients want to write to a given key. The state store clients need a mechanism to lock the key such that only one client at a time can modify a given key.

An example of this scenario occurs in active and standby systems. There could be two clients that both perform the same operation, and the operation could include the same set of state store keys. At a given time, one of the clients is active and the other is standing by to immediately take over if the active system hangs or crashes. Ideally, only one client should write to the state store at a given time. However, in distributed systems it's possible that both clients might behave as if they're active, and they might simultaneously try to write to the same keys. This scenario creates a race condition.

The state store provides mechanisms to prevent this race condition by using fencing tokens. For more information about fencing tokens, and the class of race conditions they're designed to guard against, see this article.

Obtain a fencing token

This example assumes that we have the following elements:

• Client1 and Client2. These clients are state store clients that act as an active and standby pair.
• LockName. The name of a key in the state store that acts as the lock.
• ProtectedKey. The key that needs to be protected from multiple writers.

The clients attempt to get a lock as the first step. They get a lock by doing a SET LockName {CLIENT-NAME} NEX PX {TIMEOUT-IN-MILLISECONDS}. Recall from Set Options that the NEX flag means that the SET succeeds only if one of the following conditions is met:

• The key was empty
• The key's value is already set to <value> and PX specifies the timeout in milliseconds.

Assume that Client1 goes first with a request of SET LockName Client1 NEX PX 10000. This request gives it ownership of LockName for 10,000 milliseconds. If Client2 attempts a SET LockName Client2 NEX ... while Client1 owns the lock, the NEX flag means the Client2 request fails. Client1 needs to renew this lock by sending the same SET command used to acquire the lock, if Client1 wants to continue ownership.

Use the fencing tokens on SET requests

When Client1 successfully does a SET ("AquireLock") on LockName, the state store returns the version of LockName as a Hybrid Logical Clock (HLC) in the MQTT5 user property __ts.

When a client performs a SET request, it can optionally include the MQTT5 user property __ft to represent a "fencing token". The __ft is represented as an HLC. The fencing token associated with a given key-value pair provides lock ownership checking. The fencing token can come from anywhere. For this scenario, it should come from the version of LockName.

The following diagram shows the process of Client1 doing a SET request on LockName:

Next, Client1 uses the __ts property (Property=1696374425000:1:StateStore) unmodified as the basis of the __ft property in the request to modify ProtectedKey. Like all SET requests, the client must set the __ts property of ProtectedKey.

The following diagram shows the process of Client1 doing a SET request on ProtectedKey:

If the request succeeds, from this point on ProtectedKey requires a fencing token equal to or greater than the one specified in the SET request.

Fencing Token Algorithm

The state store accepts any HLC for the __ts of a key-value pair, if the value is within the max clock skew. However, the same isn't true for fencing tokens.

The state store algorithm for fencing tokens is as follows:

• If a key-value pair doesn't have a fencing token associated with it and a SET request sets __ft, the state store stores the associated __ft with the key-value pair.
• If a key-value pair has a fencing token associated with it:
  • If a SET request didn't specify __ft, reject the request.
  • If a SET request specified a __ft that has an older HLC value than the fencing token associated with the key-value pair, reject the request.
  • If a SET request specified a __ft that has an equal or newer HLC value than the fencing token associated with the key-value pair, accept the request. The state store updates the key-value pair's fencing token to be the one set in the request, if it's newer.

After a key is marked with a fencing token, for a request to succeed, DEL and VDEL requests also require the __ft property to be included. The algorithm is identical to the previous one, except that the fencing token isn't stored because the key is being deleted.

Client behavior

These locking mechanisms rely on clients being well-behaved. In the previous example, a misbehaving Client2 couldn't own the LockName and still successfully perform a SET ProtectedKey by choosing a fencing token that is newer than the ProtectedKey token. The state store isn't aware that LockName and ProtectedKey have any relationship. As a result, state store doesn't perform validation that Client2 actually owns the value.

Clients being able to write keys for which they don't actually own the lock, is undesirable behavior. You can protect against such client misbehavior by correctly implementing clients and using authentication to limit access to keys to trusted clients only.

Notifications

Clients can register with the state store to receive notifications of keys being modified. Consider the scenario where a thermostat uses the state store key {thermostatName}\setPoint. Other state store clients can change this key's value to change the thermostat's setPoint. Rather than polling for changes, the thermostat can register with the state store to receive messages when {thermostatName}\setPoint is modified.

KEYNOTIFY request messages

State store clients request the state store monitor a given keyName for changes by sending a KEYNOTIFY message. Just like all state store requests, clients PUBLISH a QoS1 message with this message via MQTT5 to the state store system topic statestore/v1/FA9AE35F-2F64-47CD-9BFF-08E2B32A0FE8/command/invoke.

The request payload has the following form:

KEYNOTIFY<CR><LF>
{keyName}<CR><LF>
{optionalFields}<CR><LF>

Where:

• KEYNOTIFY is a string literal specifying the command.
• {keyName} is the key name to listen for notifications on. Wildcards aren't currently supported.
• {optionalFields} The currently supported optional field values are:
  • {STOP} If there's an existing notification with the same keyName and clientId as this request, the state store removes it.

The following example output shows a KEYNOTIFY request to monitor the key SOMEKEY:

*2<CR><LF>
$9<CR><LF>
KEYNOTIFY<CR><LF>
$7<CR><LF>
SOMEKEY<CR><LF>

KEYNOTIFY response message

Like all state store RPC requests, the state store returns its response to the Response Topic and uses the Correlation Data properties specified from the initial request. For KEYNOTIFY, a successful response indicates that the state store processed the request. After the state store successfully processes the request, it either monitors the key for the current client, or stops monitoring.

On success, the state store's response is the same as a successful SET.

+OK<CR><LF>

If a client sends a KEYNOTIFY SOMEKEY STOP request but the state store isn't monitoring that key, the state store's response is the same as attempting to delete a key that doesn't exist.

:0<CR><LF>

Any other failure follows the state store's general error reporting pattern:

-ERR: <DESCRIPTION OF ERROR><CR><LF>

KEYNOTIFY notification topics and lifecycle

When a keyName being monitored via KEYNOTIFY is modified or deleted, the state store sends a notification to the client. The topic is determined by convention - the client doesn't specify the topic during the KEYNOTIFY process.

The topic is defined in the following example. The clientId is an upper-case hex encoded representation of the MQTT ClientId of the client that initiated the KEYNOTIFY request and keyName is a hex encoded representation of the key that changed.

clients/statestore/v1/FA9AE35F-2F64-47CD-9BFF-08E2B32A0FE8/{clientId}/command/notify/{keyName}

As an example, MQ publishes a NOTIFY message sent to client-id1 with the modified key name SOMEKEY to the topic:

clients/statestore/v1/FA9AE35F-2F64-47CD-9BFF-08E2B32A0FE8/636C69656E742D696431/command/notify/534F4D454B4559`

A client using notifications should SUBSCRIBE to this topic and wait for the SUBACK to be received before sending any KEYNOTIFY requests so that no messages are lost.

If a client disconnects, it must resubscribe to the KEYNOTIFY notification topic and resend the KEYNOTIFY command for any keys it needs to continue monitoring. Unlike MQTT subscriptions, which can be persisted across a nonclean session, the state store internally removes any KEYNOTIFY messages when a given client disconnects.

KEYNOTIFY notification message format

When a key being monitored via KEYNOTIFY is modified, the state store will PUBLISH a message to the notification topic following the format to state store clients registered for the change.

NOTIFY<CR><LF>
{operation}<CR><LF>
{optionalFields}<CR><LF>

The following details are included in the message:

• NOTIFY is a string literal included as the first argument in the payload, indicating a notification arrived.
• {operation} is the event that occurred. Currently these operations are:
  • SET the value was modified. This operation can only occur as the result of a SET command from a state store client.
  • DEL the value was deleted. This operation can occur because of a DEL or VDEL command from a state store client.
• optionalFields
  • VALUE and {MODIFIED-VALUE}. VALUE is a string literal indicating that the next field, {MODIFIED-VALUE}, contains the value the key was changed to. This value is only sent in response to keys being modified because of a SET.

The following example output shows a notification message sent when the key SOMEKEY is modified to the value abc, with the VALUE included because the initial request specified the GET option:

*4<CR><LF>
$6<CR><LF>
NOTIFY<CR><LF>
$3<CR><LF>
SET<CR><LF>
$5<CR><LF>
VALUE<CR><LF>
$3<CR><LF>
abc<CR><LF>

Related content

• MQTT broker overview
• Develop with MQTT broker