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+## Message Store Service
+The *Message Store Service* is a simple REST service that stores messages forwarded from one or more Relayers, and serves them to clients upon request. 
+This service is an essential component for protocols such as GPACT that rely on the ability of off-chain components to query for the state of specific messages.
+
+### Features
+- Store messages forwarded by Relayers
+- Retrieve messages based on client request
+- Aggregates proof information from multiple Relayers, if required
+- Manage the lifecycle of messages if required (i.e. expiry and removal of messages)
+
+### Design and Specification
+The *Message Store Service* consumes events that have been processed by Relayers. 
+The service can be configured to store all messages that pass through a Relayer or only a subset that require persistence (e.g. GPACT events).
+A [specialised message dispatcher]("TODO") is responsible for forwarding messages to the store by interacting with its API.
+Relayers can either create a message if it is not already in the Message Store, or update it with their own attestation (or other proof), if the message already exists.
+
+The service is responsible for ensuring that all create and update operations on messages can only be performed by authorised Relayers.
+The specific authentication and authorisation mechanism to use is TBD, however the scheme should minimise the level of configuration required on the Relayer services.
+This is because a Relayer might in future work with multiple Message Stores, some of which could be dynamically specified on a per-event basis.
+This requires that the amount of context information that a Relayer requires when interacting with a *Message Store* be minimal.
+
+<p align="center">
+<img src="images/message-store-service.png" width="700"/>
+</p>
+<p align="center">Figure 1: Relayer interaction with Message Store Service</p>
+
+The API for the *Message Store Service* is detailed in [this specification document](spec/message-store-service-api.yml).
+It is defined using the [Open API Specification format](https://swagger.io/specification/) and can be displayed using the [Swagger UI tool](https://swagger.io/tools/swagger-ui/).
+
+#### TODOs
+- [ ] Security
+- [ ] Deployment considerations
+- [ ] Proof aggregation
+- [ ] Data Store description
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+swagger: '2.0'
+info:
+  description: >-
+    A service that stores and manages blockchain events forwarded by one or more
+    Relayer services. The blockchain events are stored in the canonical data
+    format employed by Relayer components.
+  version: 0.1.0
+  title: Relayer Message Store Service
+tags:
+  - name: messages
+    description: >-
+      Represents blockchain events in the canonical data format used by
+      Relayers.
+  - name: events
+    description: Represents raw blockchain events emitted by a network.
+schemes:
+  - https
+paths:
+  '/messages/{id}':
+    put:
+      tags:
+        - messages
+      summary: Adds or updates a Relayer message representing a blockchain event.
+      description: >-
+        The endpoint is used to submit (and create) a message if it does not
+        already exist in the store. If it does, the existing message is updated
+        to include the additional proof data of the submitting Relayer. Only
+        Relayers can add or update messages.
+      operationId: addOrUpdateMessage
+      consumes:
+        - application/json
+      produces:
+        - application/json
+      parameters:
+        - in: body
+          name: body
+          description: Message object to add to the store
+          required: true
+          schema:
+            $ref: '#/definitions/Message'
+        - name: id
+          in: path
+          description: ID of message to return
+          required: true
+          type: string
+      responses:
+        '200':
+          description: Exising message updated successfully
+        '201':
+          description: New message created successfully
+        '400':
+          description: Invalid input
+        '401':
+          description: Caller is not authorised to perform this operation
+      security:
+        - api_key: []
+    get:
+      tags:
+        - messages
+      summary: 'Retrieve the message associated with the given ID, if it exists.'
+      description: >-
+        Returns the message object associated with the given ID, if it exists in
+        the store.
+      operationId: getMessageById
+      produces:
+        - application/json
+      parameters:
+        - name: id
+          in: path
+          description: ID of message to return
+          required: true
+          type: string
+      responses:
+        '200':
+          description: Successful retrieval of message
+          schema:
+            $ref: '#/definitions/Message'
+        '400':
+          description: Invalid ID supplied
+        '404':
+          description: Message not found
+      security:
+        - api_key: []
+  '/messages/{id}/proofs':
+    get:
+      tags:
+        - messages
+      summary: >-
+        Retrieve all proofs that have been recorded for the message with the
+        given ID.
+      description: >-
+        Returns all proofs (e.g. attestations) that have been recorded for the
+        message with the given ID.
+      operationId: getProofs
+      produces:
+        - application/json
+      parameters:
+        - name: id
+          in: path
+          description: ID of message to return
+          required: true
+          type: string
+      responses:
+        '200':
+          description: Successful retrieval of message proofs
+          schema:
+            type: array
+            items:
+              $ref: '#/definitions/Proof'
+        '400':
+          description: Invalid ID supplied
+        '404':
+          description: Message not found
+      security:
+        - api_key: []
+    post:
+      tags:
+        - messages
+      summary: Record a new proof for message.
+      description: Records a new proof for the message with a given ID.
+      operationId: addProof
+      consumes:
+        - application/json
+      produces:
+        - application/json
+      parameters:
+        - in: body
+          name: body
+          description: Message proof object to add to the store
+          required: true
+          schema:
+            type: array
+            items:
+              $ref: '#/definitions/Proof'
+        - name: id
+          in: path
+          description: ID of message to return
+          required: true
+          type: string
+      responses:
+        '200':
+          description: Proof successfuly added to message
+        '400':
+          description: Invalid ID or proof object supplied
+        '401':
+          description: Caller is not authorised to perform this operation
+        '404':
+          description: Message not found
+        '409':
+          description: Given message proof was already recorded
+        '422':
+          description: Validation of attestation failed
+      security:
+        - api_key: []
+  '/events/{networkId}/{blockNumber}/{txIndex}/{logIndex}':
+    get:
+      tags:
+        - events
+      summary: >-
+        Retrieve's a message that represents the blockchain event identified
+        with the given path parameters.
+      description: >-
+        Returns a message that represents the identified blockchain event, if
+        one exists. This endpoint offers a convinient way for clients that only
+        know identifying details of an event (and not the associated Relayer
+        message), to interact with messages in the store.
+      operationId: getEvent
+      produces:
+        - application/json
+      parameters:
+        - name: networkId
+          in: path
+          description: ID of network in which the event was generated
+          required: true
+          type: string
+        - name: blockNumber
+          in: path
+          description: Block number that contains this event
+          required: true
+          type: string
+        - name: txIndex
+          in: path
+          description: Index number within the block for the transaction that generates the event
+          required: true
+          type: string
+        - name: logIndex
+          in: path
+          description: Index number of the event within the block
+          required: true
+          type: string
+      responses:
+        '200':
+          description: Successful retrieval of associated message
+          schema:
+            $ref: '#/definitions/Message'
+        '400':
+          description: One or more invalid path parameters provided
+        '404':
+          description: Message not found
+      security:
+        - api_key: []
+securityDefinitions:
+  api_key:
+    type: apiKey
+    name: api_key
+    in: header
+definitions:
+  Proof:
+    type: object
+    required:
+      - type
+      - created
+      - proof
+    properties:
+      type:
+        type: string
+      created:
+        type: string
+      proof:
+        type: string
+  NetworkAddress:
+    type: object
+    required:
+      - networkId
+      - bridgeAddress
+      - contractAddress
+    properties:
+      networkId:
+        type: string
+      contractAddress:
+        type: string
+      bridgeAddress:
+        type: string
+  Meta:
+    type: object
+    required:
+      - timestamp
+      - version
+    properties:
+      timestamp:
+        type: string
+      version:
+        type: string
+  Message:
+    type: object
+    required:
+      - id
+    properties:
+      id:
+        type: string
+      meta:
+        $ref: '#/definitions/Meta'
+      destination:
+        $ref: '#/definitions/NetworkAddress'
+      source:
+        $ref: '#/definitions/NetworkAddress'
+      proofs:
+        type: array
+        items:
+          $ref: '#/definitions/Proof'
+      payload:
+        type: string