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Makara

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Makara is generic master/slave proxy. It handles the heavy lifting of managing, choosing, blacklisting, and cycling through connections. It comes with an ActiveRecord database adapter implementation.

Warning:

There is a potential performance issue when used alongside certain versions of newrelic/rpm, Issue #59.

Any newrelic_rpm < 3.11.2 and >= 3.7.2 will have the regression. I'd recommend upgrading newrelic_rpm to avoid the problem.

Installation

gem 'makara', github: 'taskrabbit/makara', tag: 'v0.3.x'

Basic Usage

If you're only interested in the ActiveRecord database adapter... here you go.

Makara provides a base proxy class which you should inherit from. Your proxy connection class should implement a connection_for instance method which will be provided with an individual configuration and expect a real connection back.

class MyAwesomeSqlProxy < ::Makara::Proxy
  def connection_for(config)
    ::Sql::Client.new(config)
  end
end

Next, you need to decide which methods are proxied and which methods should be sent to all underlying connections:

  # within MyAwesomeSqlProxy
  hijack_method :select, :ping
  send_to_all :connect, :reconnect, :disconnect, :clear_cache

Assuming you don't need to split requests between a master and a slave, you're done. If you do need to, implement the needs_master? method:

  # within MyAwesomeSqlProxy
  def needs_master?(method_name, args)
    return false if args.empty?
    sql = args.first
    sql !~ /^select/i
  end

This implementation will send any request not like "SELECT..." to a master connection. There are more methods you can override and more control over blacklisting - check out the makara database adapter for examples of advanced usage.

Config Parsing

Makara comes with a config parser which will handle providing subconfigs to the connection_for method. Check out the ActiveRecord database.yml example below for more info.

Context

Makara handles stickyness by keeping track of a context (sha). In a multi-instance environment it persists a context in a cache. If Rails is present it will automatically use Rails.cache. You can provide any kind of store as long as it responds to the methods required in lib/makara/cache.rb.

Makara::Cache.store = MyRedisCacheStore.new

To handle persistence of context across requests in a Rack app, makara provides a middleware. It lays a cookie named _mkra_ctxt which contains the current master context. If the next request is executed before the cookie expires, master will be used. If something occurs which naturally requires master on the second request, the context is changed and stored again.

Changing Context

If you need to change the makara context, releasing any stuck connections, all you have to do is:

ctx = Makara::Context.generate # or any unique sha
Makara::Context.set_current ctx

A context is local to the curent thread of execution. This will allow you to stick to master safely in a single thread in systems such as sidekiq, for instance.

Forcing Master

If you need to force master in your app then you can simply invoke stick_to_master! on your connection:

write_to_cache = true # or false
proxy.stick_to_master!(write_to_cache)

Skipping the Stickiness

If you're using the sticky: true configuration and you find yourself in a situation where you need to write information through the proxy but you don't want the context to be stuck to master, you should use a without_sticking block:

proxy.without_sticking do
  # do my stuff that would normally cause the proxy to stick to master
end

Logging

You can set a logger instance to ::Makara::Logging::Logger.logger and Makara will log how it handles errors at the Proxy level.

Makara::Logging::Logger.logger = ::Logger.new(STDOUT)

ActiveRecord Database Adapter

So you've found yourself with an ActiveRecord-based project which is starting to get some traffic and you realize 95% of you DB load is from reads. Well you've come to the right spot. Makara is a great solution to break up that load not only between master and slave but potentially multiple masters and/or multiple slaves.

By creating a makara database adapter which simply acts as a proxy we avoid any major complexity surrounding specific database implementations. The makara adapter doesn't care if the underlying connection is mysql, postgresql, etc it simply cares about the sql string being executed.

What goes where?

Any SELECT statements will execute against your slave(s), anything else will go to master. The only edge case is SET operations which are sent to all connections. Execution of specific methods such as connect!, disconnect!, and clear_cache! are invoked on all underlying connections.

Errors / blacklisting

Whenever a node fails an operation due to a connection issue, it is blacklisted for the amount of time specified in your database.yml. After that amount of time has passed, the connection will begin receiving queries again. If all slave nodes are blacklisted, the master connection will begin receiving read queries as if it were a slave. Once all nodes are blacklisted the error is raised to the application and all nodes are whitelisted.

Database.yml

Your database.yml should contain the following structure:

production:
  adapter: 'mysql2_makara'
  database: 'MyAppProduction'
  # any other standard AR configurations

  # add a makara subconfig
  makara:

    # the following are default values
    blacklist_duration: 5
    master_ttl: 5
    master_strategy: round_robin
    sticky: true

    # list your connections with the override values (they're merged into the top-level config)
    # be sure to provide the role if master, role is assumed to be a slave if not provided
    connections:
      - role: master
        host: master.sql.host
      - role: slave
        host: slave1.sql.host
      - role: slave
        host: slave2.sql.host

Let's break this down a little bit. At the top level of your config you have the standard adapter choice. Currently the available adapters are listed in lib/active_record/connection_adapters/. They are in the form of #{db_type}_makara where db_type is mysql, postgresql, etc.

Following the adapter choice is all the standard configurations (host, port, retry, database, username, password, etc). With all the standard configurations provided, you can now provide the makara subconfig.

The makara subconfig sets up the proxy with a few of its own options, then provides the connection list. The makara options are:

  • blacklist_duration - the number of seconds a node is blacklisted when a connection failure occurs
  • sticky - if a node should be stuck to once it's used during a specific context
  • master_ttl - how long the master context is persisted. generally, this needs to be longer than any replication lag
  • master_strategy - use a different strategy for picking the "current" node: failover will try to keep the same one until it is blacklisted. The default is round_robin which will cycle through available ones.
  • connection_error_matchers - array of custom error matchers you want to be handled gracefully by Makara (as in, errors matching these regexes will result in blacklisting the connection as opposed to raising directly).

Connection definitions contain any extra node-specific configurations. If the node should behave as a master you must provide role: master. Any previous configurations can be overridden within a specific node's config. Nodes can also contain weights if you'd like to balance usage based on hardware specifications. Optionally, you can provide a name attribute which will be used in sql logging.

connections:
  - role: master
    host: mymaster.sql.host
    blacklist_duration: 0

  # implicit role: slave
  - host: mybigslave.sql.host
    weight: 8
    name: Big Slave
  - host: mysmallslave.sql.host
    weight: 2
    name: Small Slave

In the previous config the "Big Slave" would receive ~80% of traffic.

DATABASE_URL

Connections may specify a url parameter in place of host, username, password, etc.

connections:
  - role: master
    blacklist_duration: 0
    url: 'mysql2://db_username:db_password@localhost:3306/db_name'

We recommend, if using environmental variables, to interpolate them via ERb.

connections:
  - role: master
    blacklist_duration: 0
    url: <%= ENV['DATABASE_URL_MASTER'] %>
  - role: slave
    url: <%= ENV['DATABASE_URL_SLAVE'] %>

Important: Do NOT use ENV['DATABASE_URL'], as it inteferes with the the database configuration initialization and may cause Makara not to complete the configuration. For the moment, it is easier to use a different ENV variable than to hook into the database initialization in all the supported Rails.

For more information on url parsing, as used in ConfigParser, see:

Custom error matchers:

To enable Makara to catch and handle custom errors gracefully (blacklist the connection instead of raising directly), you must add your custom matchers to the connection_error_matchers setting of your config file, for example:

production:
  adapter: 'mysql2_makara'

  makara:
    blacklist_duration: 5
    connection_error_matchers:
      - !ruby/regexp '/^ActiveRecord::StatementInvalid: Mysql2::Error: Unknown command:/'
      - '/Sql Server Has Gone Away/'
      - 'Mysql2::Error: Duplicate entry'

You can provide strings or regexes. In the case of strings, if they start with / and end with / they will be converted to regexes when evaluated. Strings that don't start and end with / will get evaluated with standard comparison.

Common Problems / Solutions

On occasion your app may deal with a situation where makara is not present during a write but a read should use master. In the generic proxy details above you are encouraged to use stick_to_master! to accomplish this. Here's an example:

# some third party creates a resource in your db, slave replication may not have completed yet
# ...
# then your app is told to read the resource.
def handle_request_after_third_party_record_creation
  CreatedResourceClass.connection.stick_to_master!
  CreatedResourceClass.find(params[:id]) # will go to master
end

Similarly, if you have a third party service which will conduct a generic request against your Rack app, you can force master via a query param:

def send_url_to_third_party
  context = Makara::Context.get_current
  ThirdParty.read_from_here!("http://mysite.com/path/to/resource?_mkra_ctxt=#{context}")
end

Todo

  • Cookie based cache store?
  • More real world examples