mmmbbb

A Message Bus for 6RiverSystems.

This is, in fact, a near drop-in replacement for the Google PubSub emulator. Specifically it implements most of the PubSub gRPC API, and specifically a superset of what the emulator does, sufficient for the needs of 6RS.

Running it

For Rivs

This will be run automatically by 6mon.

For others

You can build and run the mmmbbb app locally, or you can use our published Docker image 6river/mmmbbb-service. The latter is simpler.

The docker image defaults to using a SQLite back-end, which is stored in a /data volume in the container. This is done to minimize the amount of setup required, but it should be noted that the PostgreSQL backend has considerably better performance.

In its simplest form, you can use:

docker run --rm --publish 8084-8085:8084-8085/tcp 6river/mmmbbb-service

If you want to keep the SQLite database across runs, add --volume /local/path:/data to have the database kept in /local/path/mmmbbb.sqlite.

Initializing a PostgreSQL Database

The application will create all the tables it needs automatically, as long as it can connect to the database.

You can further simplify setup by including the CREATE_DB_VIA environment variable. If the DATABASE_URL is otherwise valid, but the target database doesn't exist, and this environment variable is set, the app will try to connect to this alternate database and issue a CREATE DATABASE for its target database. If this succeeds, it will switch back to its own database and continue with startup.

Configuration

Environment variables

• NODE_ENV
  • Useful values are test, development, or production
  • Determines defaults for logging and the database to use
• DATABASE_URL
  • Supported values are postgres://user:password@host:port/database or sqlite:///path/to/file.sqlite (note the number of slashes!). See below for extra notes for using SQLite.
• LOG_LEVEL
  • Supported values are trace, debug, info, warn, error, fatal, panic (i.e. the level names from zerolog)
• PORT
  • Sets the base port for the application. This is where it listens for normal HTTP connections for metrics and the OAS UI.
  • The gRPC port will be this plus one
• CREATE_DB_VIA
  • See above regarding PostgreSQL initialization

Supported persistence backends

Both PostgreSQL and SQLite are supported backends. With SQLite, only file-based storage is currently supported, not memory-based storage, due to issues with concurrency, locking, and some implementation issues with the Go SQLite driver.

To select a backend, set the DATABASE_URL appropriately (see above on environment variables).

SQLite as a backend currently requires explicitly specifying several extra parameters in the DATABASE_URL:

?_fk=true&_journal_mode=wal&cache=private&_busy_timeout=10000&_txlock=immediate

What is and isn't implemented

The bulk of the gRPC API is implemented. If your application only sends JSON payloads, and works with the emulator Google provides as part of the Google Cloud SDK as of June 2021, it should work with this one too.

Supported Features Beyond Google's Emulator

Several features are supported here that Google's emulator does not support (as of June 2021):

• Server driven delivery backoff (but see note below on how this differs from Google's production implementation)
• Subscription filters
• Dead letter topics for subscriptions
• Ordered message delivery to subscriptions

Features that are not implemented

• Region settings
• KMS settings
• Custom ACK deadlines
• ACK'd message retention
  • While you cannot configure this, there is a limited retention that is always active, see below on the partial support for seek to time
• Authenticating PUSH delivery
• The Schema API and any PubSub settings related to it
• Authentication settings for Push subscriptions
• Authenticating to the service itself
• Detaching subscriptions
• Subscription Snapshots and seeking to them
  • Seeking to a time is partially supported, but see below for notes
• Changing message retry backoff settings for a subscription may not fully take effect when actively streaming messages from it (including from an HTTP Push configuration)

Features that are different

• Some default timeouts & expiration delays may have different defaults from the Google ones, though they should all still be suitable for development purposes
• You cannot actually turn off the exponential backoff for delivery retries, though you can change its settings. If configure a subscription without this enabled, a default set of backoff settings will be used and it will still be active under the hood, even though it will appear disabled in the gRPC API.
• All published messages must (currently) have a JSON payload, not arbitrary binary data. This restriction may be lifted in the future.
• Some activities the Google emulator considers an error, mmmbbb does not. For example, several subscription settings can be modified via the gRPC API after subscription creation in mmmbbb, but not in Google's products. Google treats an HTTP Push endpoint that returns content with a 201 No Content response as an error (NACK), mmmbbb does not.
• This is a clean room implementation, based only on documented behavior. As such, some corner cases where Google does not describe what their system does in their API documentation may be have differently with this implementation.
  • Example: Google does not currently document what happens if you delete a topic that is referenced as the dead letter topic for some Subscription.
• Seeking a subscription to a time doesn't require enabling message retention, since mmmbbb always retains messages to a limited extent. While seeking to a time outside the limited automatic retention will not produce an error, neither will it resurrect messages that have been permanently deleted.

Database model

The data model uses four tables: topics, subscriptions, messages, and deliveries.

Topics

Each PubSub topic is represented by a row in the topics table. When topics are deleted, it is initially a "soft" delete, marking the row inactive. A background process will eventually prune it once all the dependent rows in other tables have been pruned as well.

In addition to helping performance, the background pruning model also allows objects to be kept around for some time after they are deleted / completed for debug / diagnostic inspections.

Subscriptions

Each PubSub subscription is represented by a row in the subscriptions table. Like with topics, deleting a subscription is initially a "soft" delete, followed up by a background pruning process. Subscriptions are attached to topics by an internal id, not by name, mirroring the behavior of the Google system.

Messages

Every published message becomes a row in the messages table, linked to the topic on which it was published.

Deliveries

When a message is published, a row is added to the deliveries table for each active subscription in the topic. Message ack/nack operations update the corresponding row. Like with other entities, completed or expired deliveries are not immediately removed, but instead get a "soft" marker, and are deleted later by a background pruning process.

Fault Injection

The application uses the fault injection "framework" from gosix to allow any of the gRPC calls to have faults (errors) injected. The state of this is queried via the GET /faults endpoint, and new injections can be added via the POST /faults/inject endpoint. For more details on this, check the Swagger-UI at /oas-ui/, or the gosix documentation on the gRPC fault interceptor.

Examples:

To cause one call to the subscriber StreamingPull API to fail, for a specific subscription, you could inject the following fault configuration:

{
  "operation": "StreamingPull",
  "parameters": {
    "subscription": "projects/foo/subscriptions/bar"
  },
  "count": 1
}

To cause one call to the Publish API to fail, for a specific topic, you could inject the following fault configuration:

{
  "operation": "Publish",
  "parameters": {
    "topic": "projects/foo/topics/bar"
  },
  "count": 1
}