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er: erlang redis

Status

er is production ready.

er is command complete with redis-2.8.5.

er is adding new Redis features soon (Sentinel, Cluster, ...).

Code Guide

src/er.lfe and src/erp.lfe are where redis commands get created.

er.lfe creates a module where the first parameter of all commands is the redis connection (the conenction is either a er_redis PID or er_pool name):

    {ok, Client} = er_redis:connect().
    er:set(Client, <<"chevron">>, <<"locked">>).

er_pool.erl gives you a centrally managed connection pool of redis clients. Create a named pool, then use regular er commands against it. If you use a command requiring exclusive use of a client (b[lr]pop, brpoplpush, subscribe, watch, etc), the client is taken out of the general pool and reserved for your individual use. When the client is done with exclusive operations, the client is returned to the general connection pool.

Connect and use anonymous pid

    {ok, Client} = er_pool:start_link().
    er:set(Client, italian, greyhound).
    <<"greyhound">> = er:get(Client, italian).

Connect and use named pool (no need for pid tracking)

    er_pool:start_link(userdb).
    er:set(userdb, hash_value_1, erlang:md5(<<"hash this">>).
    er:lpush(userdb, updates, hash_value_1).

Transaction support (multi/exec/discard)

    er_pool:start_link(txn_readme_test).
    TxnFun = fun(Cxn) ->
               er:setnx(Cxn, hello, there),
               er:incr(Cxn, "counter:uid")
             end,
    er:er_transaction(txn_readme_test, TxnFun).

Blocking Pop

Blocking pop blocks the current process until Timeout (600 seconds below) or until an item becomes available. Blocking operations return the atom nil on timeout.

    er_pool:start_link(workqueue_pool).
    er:blpop(workqueue_pool, blocked_key, 600).

PubSub

    er_pool:start_link(announcements).
    % Subscribe to key `another`.  Returns a pid and subscribe validation.
    {SubClientPid, [subscribe, <<"another">>, 1]} = er:subscribe(announcements, another).

    % To receive published messages, run a blocking receive.
    % er_next blocks your current process until something is published.
    % You can run er:er_next(SubClientPid) in a loop to consume all published messages.
    [message, <<"bob">>, PublishedMessage] = er:er_next(SubClientPid).

    % Clean up when you are done to avoid leaking processes and sockets:
    SubClientPid ! shutdown.

For per-command usage examples, see test/er_tests.erl.

See files include/{utils,utils-macro,redis-cmds,redis-return-types}.lfe:

  • utils.lfe - shared utility functions
  • utils-macro.lfe - includes the macro which generates return value macros
  • redis-cmds.lfe - all redis commands and their return type handlers
  • redis-return-types.lfe - an alternate format of specifying redis return types. Contains functions for converting redis return values to native types.

eru

Module eru has utility functions to help you through your redis adventures.

eru:dump(Server) -- dump all keys and values from a server regardless of type.

eru:dump(Server, Key) -- print the key and value from a server regardless of type.

eru:er_key/1, eru:er_key/2, ..., eru:er_key/32 -- create a formatted redis key. Accepts atoms, integers, lists, and binaries. example: eru:er_key(this, <<"is">>, "a", gr, 8, key) = <<"this:is:a:gr:8:key">>

Building

Download LFE:

    ./rebar get-deps

Build:

    ./rebar compile

Testing

NB: Tests run against a redis on port 6991. Running tests will DELETE ALL DATA on your port 6991 redis. rebar eunit skip_deps=true suite=er -v rebar eunit skip_deps=true suite=er_concurrency_tests -v

Next Steps

In no specific order:

  • More comprehensive test cases for er/erp
  • More features
    • bidirectional mnesia sync?
    • redis as erlang term store vs. redis as generic store
    • generate modules for pre-defined common accessors

When to use er

Use er when you want to access redis from erlang.

er converts redis return types to erlang-friendly terms. redis 1 and 0 responses are converted to true/false atoms when appropriate. redis binary results are returned as erlang binaries. redis integer results are returned as erlang numbers.

er aims to be the most semantically correct erlang redis client with a sane translation layer between how redis views the world and how erlang views the world.

A note on testing

er is just an interface to redis. We're more interested in testing the border between redis and erlang than testing redis itself. We don't need the entire redis test suite imported yet. (Though, if you want to write a redis tcl test suite to eunit translator go right ahead.)

Testing is the exploratory way er becomes more correct over time. Find an untested redis command, use it, see the result. Did it return the right value with the proper type? Numbers got returned as N and not <<"N">>? True/False got returned as true/false and not 1/0?

If the return types are wrong, jump into include/redis-cmds.lfe and update the return type for the command you are testing.

If the redis command has optional arguments you may need to create multiple functions with varying airity. See anything using withscores as an example in include/redis-cmds.lfe.

You can implement the same redis command multiple times but with different return types by specifying a unique function name. See hgetall_p and hgetall_k in include/redis-cmds.lfe for an example.

Testing new commands may require modifying the return type processing functions themselves in include/redis-return-types.lfe. Jump in and make things better.

Issues during testing to keep in mind:

  • rebar does not rebuild based on lfe includes. Delete ebin/ to rebuild.
  • Does the return type make sense?
    • No always-integers returned in binaries
    • Redis errors throw exceptions instead of returning error tuples
  • Did your input get auto-converted to a binary and sent to redis?
    • Can you read the same value back from redis?
      • Everything-to-binary conversion functions are at the end of src/er_redis.erl
  • Test creation, read, update, read
  • Test edge cases and boundary conditions
  • Test error conditions
  • Test impossibles (see if you can crash redis)
    • e.g. inf + -inf, nan + inf, -inf + 3, etc.
  • Will concurrent testing or fuzzing help? Add test suites as necessary.

Contributions

Want to help? Patches welcome.

  • Poke around in test/er_tests.erl and add some missing tests. Ideas:
    • Figure out how to implement sort nicely.
    • Make info output pretty
    • Test eru
    • Test long-lived things
      • publish/subscribe
      • multi/exec/discard (what should the interface for those look like?)
      • monitor
  • Write something to parse include/er-cmds.lfe and output docs for return types.
  • Make a nicer interface to pubsub and blocking calls.
    • Spawn an owner process for them you send/receive erlang messages to/from?
      • named? unnamed? how long lived? auto-reconnect on errors/timeout?
  • Figure out why markdown isn't auto-linking my User/Project@Commit references like the docs say it should
  • Find a bug. Fix a bug.
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