Difference between revisions of "Zaqar/bp/placement-service"

Revision as of 14:58, 6 August 2013

Overview

• 

  Placement Service Draft

Marconi depends on storage for maintaining state on messages. However, as it is designed now, this is a scaling bottleneck because at most one storage node can be specified per Marconi instance. This can be partially offset by taking advantage of built-in sharding and replication for storage providers like MongoDB, but many other storage engines do not provide such a system. Furthermore, the migration of queues from one storage node to another is not supported cleanly by most storage providers. More intelligent data migration can be performed by taking into account that Marconi operates at the level of queues.

The placement service aims to address this by providing a means to specify dynamically where storage for Marconi resources should be performed.

Responsibilities

• Maintaining a catalogue, a mapping of (Project + Queue) => storage node locations
• Handling queue creation transparently
• Handling migration transparently
• Handling deletion of queues transparently

Transparency

• User transparency: availability and use of the Marconi service must not be interrupted when a migration is taking place.
• Implementation transparency: storage driver is handed a location/connection and only cares about the serialization/deserialization of data to that storage location.

Approach

A placement service consists of the following components:

• A catalogue of mappings: {queue => [read locations], queue => [write locations], queue => status}

   * where status is one of ACTIVE or MIGRATING

• A cache maintained in each Marconi instance of the mappings
• A migration service

Placement API

The placement service must handle:

• storage assignment
• cataloging
• migrations

An API must be exposed to facilitate these operations.

Storage Management

• Add node (POST /v1/storage: {'location': 'mongo1.storage.com' , 'weight': 100})
• Delete node (DELETE /v1/storage/mongo1.storage.com)
• Change weight (PUT /v1/storage/mongo1.storage.com {'weight': 50}
• Query storage (GET /v1/storage => [...], GET /v1/storage/mongo1.storage.com => {'weight': 100})

Catalogue Management

• Add entry (POST)
• Update an entry (PUT)
• Remove entry (DELETE)

Migration

This means: "move object from location A to location B". In terms of the Placement service, it must:

• Update the storage write location
• Update the storage read location
• Update the entry status ("a" => "m")

Catalogue

The catalog, as described above, maintains these mappings from queues to locations.

Marconi can continue to function in a degraded state if the placement service goes down. The following capabilities will cease to function for Marconi if the placement service goes down:

• Migration
• Queue creation

It may be possible to continue performing queue creation and deletion while the placement service is down by defaulting to the last storage location that Marconi held.

The data contained in the catalogue can be regenerated by querying each Marconi endpoint and synchronizing against each cache.

The Marconi catalogue contains a series of structures that look as follows:

{
    "{project}.{queue}" : {
        "r": [("mongo://192.168.1.105:7777", "db1"), ("mongo://192.168.1.106:7777", "db2")],
        "w": [("mongo://192.168.1.105:7777", "db1")],
        "s": "a"
    }
}

An entry in the queue is found by concatenating a project with a queue name. "r" is the collection of locations where data for a particular queue can be gathered from. "w" is the locations where new data written to this queue are stored. "s" is the state of the queue: active "a" or migrating "m". A storage location is given by a URL and a database name.

The catalogue is filled with many such documents. The local cache for a given Marconi node is populated with the entries from this cache, to avoid lookups on each request.

Adding Items to the Catalogue

Whenever a Marconi queue is created, a hook is called that transfers control the the placement service. It then becomes the placement service's responsibility to:

• Take the given project ID and namespace (queue name)
• Assign a read storage location to it
• Assign a write storage location to it
• Store these locations in the catalogue
• Return these locations to the Marconi worker

Removing Items from the Catalogue

When a Marconi queue has been deleted, a hook must be called to notify the Placement service. The placement service must then remove the entry from its catalogue and notify all subscribed Marconi listeners.

Placement Policy Engine

The placement service will use a weighted distribution to determine storage assignment for newly created queues. However, for certain tenants, we might want to assign them to special or dedicated storage.

A weighted distribution might look like:

{
    "mongo1.storage.com": 30,
    "mongo2.storage.com": 20,
    "mongo3.storage.com": 30,
    "mongo4.storage.com": 75,
    "mongo5.storage.com": 5,
}

This means that the probability of a queue being stored on mongo1 is (30)/(30 + 20 + 30 + 75 + 5) => 18.75%.

The policy engine is responsible for maintaining this list of special tenants and their preferred mappings. Whenever a request to create a queue is received, Placement iterates through this list, and if the tenant is found, the associated dedicated storage is returned. If no such tenant is found, then the assignment of storage to the queue is determined by Placement.

Marconi Cache: Using PubSub

If the Marconi catalog and the local caches are implemented using a pubsub-based transport, the invalidation procedure can be simplified. The catalog publishes on the cache channel and all listening Marconi workers update their local cache accordingly. Simple messages in the form of:

• <action> <queue> [<destination>]
• Where action is one of: delete, create, migrate
• Where destination is valid only for migrate, and if left unspecified, leaves the decision of destination up the placement service.

Possibilities here include: Redis, ZeroMQ

Placement Service API

To be filled soon: a Python transport/storage API for managing the placement service catalogue and storage nodes.

Roadmap

Phase 1: Replication and Sharding

In the first iteration of this project, the goal is to provide an easy way to replicate and shard data across dynamically allocated storage nodes. The required features are:

• Catalogue + management API
• Storage allocation + management API: static weights
• Policy engine: being able to assign dedicated storage to particular tenants
• Local catalogue cache + push consistency

Phase 2: Migration, Dynamism, and Deletion

Phase 2 introduces the ability to migrate data (read: Marconi data, queues + messages) from one storage node to another. The ability to remove entries from the catalog and also schedule them to be removed from storage is considered. Finally, dynamically controlled storage allocation is introduced for more hands-off operation. In sum:

• Migration + migration API: move data from storage node to node, set storage as read-only
• Deletion: remove data from catalog and from storage nodes
• Dynamism: monitor storage nodes and adjust weights based on node capacity and load

Phase 3: Data Affinity and Generalization

This phase optimizes and generalizes placement service. Conceptually, there's no reason placement service should serve only the needs of Marconi. Requirements:

• Data affinity: attempt to cache particular storage connections on worker nodes where certain data appears more often

   - Useful for reducing the number of cached connections

• Generalization: make the placement service usable by other services

Phase N: ???

Because the future is open, and predicting beyond this point is very difficult.

Ideas Under Consideration

Periodic Refresh

On a separate Marconi "thread", poll the catalogue service periodically (say, every 10 seconds). This actor is responsible for updating the cache. It queries the catalogue service and looks for changes.

To enable this, migrations are only allowed a granularity of 5 minutes. This helps avoid race conditions on a catalogue resource, since the migration itself triggers a state change from active to migrating for a particular queue.

Push Refresh

This approach does away with time and puts the responsibility of invalidating caches on the placement service. All Marconi nodes must maintain a listen port connected to the placement service, and whenever a migration occurs, Marconi nodes receive updates on queues that are being affected.

Deletions

Deletions take priority over migrations. If a migration is in progress for Q1, and a request to delete Q1 is made, then all messages for Q1 are deleted both from the initial storage location and the destination storage location. The migration is cancelled.

Dynamic Weight Management

The operator of the placement service can manually determine the weights of the storage locations to bootstrap the system. However, in the future, it would be preferred to dynamically update these weights based on host parameters such as:

• Storage location CPU load
• Storage location remaining capacity

This adds a level of intelligence to the placement storage layer that makes maintenance a more hands-free experience.

Connection Pooling

To be filled soon: caching strategy for storage connections at the Marconi worker level