CQErl

Native Erlang client for CQL3 over Cassandra's binary protocol v2 (a.k.a. what you want as a client for Cassandra).

Usage · Connecting · Performing queries · Query options · Batched queries · Reusable queries · Data types

Installation · Tests · License

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This project is still relatively new and is under active development, so you are welcome to contribute to it with improvements, suggestions and any issues you encounter.

At a glance

CQErl offers a simple Erlang interface to Cassandra using the latest CQL version (v3). The main features include:

• Automatic (and configurable) connection pools using pooler
• Batched queries
• Variable bindings in CQL queries (named or not)
• Automatic query reuse when including variable bindings
• Collection types support
• Tunable consistency level
• Synchronous or asynchronous queries
• Automatic compression (using lz4 or snappy if available)
• SSL support
• Pluggable authentication (as long as it's SASL-based)

CQErl was designed to be as simple as possible on your side. You just provide the configuration you want as environment variables, and ask for a new client everytime you need to perform a transient piece of work (e.g. handle a web request). You do not need to (and should not) keep a client in state for a long time. Under the hood, CQErl maintains a pool of persistent connections with Cassandra and this pattern is the best way to ensure proper load balancing of requests across the pool.

Usage

Connecting

If you installed cassandra and didn't change any configuration related to authentication or SSL, you should be able to connect like this

{ok, Client} = cqerl:new_client({}).

And close the connection like this

cqerl:close_client(Client).

1. The first argument to cqerl:new_client/2 or cqerl_new_client/1 is the node to which you wish to connect as {Ip, Port}. If empty, it defaults to {"127.0.0.1", 9042}, and Ip can be given as a string, or as a tuple of components, either IPv4 or IPv6.

2. The second possible argument (when using cqerl:new_client/2) is a list of options, that include auth (mentionned below), ssl (which is false by default, but can be set to a list of SSL options) and keyspace (string or binary). Other options include pool_max_size, pool_min_size and pool_cull_interval, which are used to configure pooler (see its documentation to understand those options).

If you've set simple username/password authentication scheme on Cassandra, you can provide those to CQErl

{ok, Client} = cqerl:new_client({}, [{auth, {cqerl_auth_plain_handler, [{"test", "aaa"}]}}]).

Since Cassandra implements pluggable authentication mechanisms, CQErl also allows you to provide custom authentication modules (here cqerl_auth_plain_handler). The options you pass along with it are given to the module's auth_init/3 as its first argument.

Using environment variables

All the options given above can be provided as environment variables, in which case they are used as default (and overridable) values to any cqerl:new_client calls. You can also provide a cassandra_nodes variable containing a list of the tuples used as the first argument to cqerl:new_client. So for example, in your app.config or sys.config file, you could have the following content:

[
  {cqerl, [
            {cassandra_nodes, [ { "127.0.0.1", 9042 } ]},
            {ssl, [ {cacertfile, "cassandra.pem"} ]},
            {auth, {cqerl_auth_plain_handler, [ {"test", "aaa"} ]}}
          ]},
]

Doing so will fire up connection pools as soon as the CQErl application is started. So when later on you call cqerl:new_client, chances are you will hit a preallocated connection (unless they're so busy that CQErl needs to fire up new ones). In fact, if you provide the cassandra_nodes environment variable, you can call cqerl:get_client/0, which chooses an available client at random.

Performing queries

Performing a query can be as simple as this:

{ok, Result} = cqerl:run_query(Client, "SELECT * FROM users;").

% Equivalent to
{ok, Result} = cqerl:run_query(Client, <<"SELECT * FROM users;">>).

% Also equivalent to
{ok, Result} = cqerl:run_query(Client, #cql_query{statement = <<"SELECT * FROM users;">>}).

It can also be performed asynchronously using

Tag = cqerl:send_query(Client, "SELECT * FROM users;"),
receive
    {result, Tag, Result} ->
        ok
end.

In situations where you do not need to wait for the response at all, it's perfectly fine to produce this sort of pattern:

{ok, Client} = cqerl:new_client(),
cqerl:send_query(Client, #cql_query{statement="UPDATE secrets SET value = null WHERE id = ?;",
                                    values=[{id, <<"42">>}]}),
cqerl:close_client(Client).

That is, you can grab a client only the send a query, then you can get rid of it. CQErl will still perform it, the difference being that no response will be sent back to you.

Here's a rundown of the possible return values

• SELECT queries will yield result of type #cql_result{} (more details below).
• Queries that change the database schema will yield result of type #cql_schema_changed{type, keyspace, table}
• Other queries will yield void if everything worked correctly.
• In any case, errors returned by cassandra in response to a query will be the return value ({error, Reason} in the synchronous case, and {error, Tag, Reason} in the asynchronous case).

#cql_result{}

The return value of SELECT queries will be a #cql_result{} record, which can be used to obtain rows as proplists and fetch more result if available

{ok, _SchemaChange} = cqerl:run_query(Client, "CREATE TABLE users(id uuid, name varchar, password varchar);"),
{ok, void} = cqerl:run_query(Client, #cql_query{
    statement = "INSERT INTO users(id, name, password) VALUES(?, ?, ?);",
    values = [
        {id, new},
        {name, "matt"},
        {password, "qwerty"}
    ]
}),
{ok, Result} = cqerl:run_query(Client, "SELECT * FROM users;").

Row = cqerl:head(Result),
Tail = cqerl:tail(Result),
{Row, Tail} = cqerl:next(Result),
1 = cqerl:size(Result),
0 = cqerl:size(Tail),
empty_dataset = cqerl:next(Tail),
[Row] = cqerl:all_rows(Result),

<<"matt">> = proplists:get_value(name, Row),
<<"qwerty">> = proplists:get_value(password, Row).

#cql_result{} can also be used to fetch more result, synchronously or asynchronously

case cqerl:has_more_pages(Result) of
    true -> {ok, Result2} = cqerl:fetch_more(Result);
    false -> ok
end,

Tag2 = cqerl:fetch_more_async(Result),
receive
    {result, Tag2, Result2} -> ok
end.

#cql_schema_changed{}

#cql_schema_changed{} is returned from queries that change the database schema somehow (e.g. ALTER, DROP, CREATE, and so on). It includes:

1. The type of change, either created, updated or dropped
2. The name of the keyspace where the change happened, as a binary
3. If applicable, the name of table on which the change was applied, as a binary

Providing options along queries

When performing queries, you can provide more information than just the query statement using the #cql_query{} record, which includes the following fields:

1. The query statement, as a string or binary

2. values for binding variables from the query statement. This is a proplists, where the keys match the column names or binding variable names in the statement, in lowercase. Special cases include providing TTL and TIMESTAMP option in statements, in which case the proplist key would be [ttl] and [timestamp] respectively.

   Also, when providing the value for a uuid-type column, you can give the value new, strong or weak, in which case CQErl will generate a random UUID (v4), with either a strong or weak number random generator.

   Finally, when providing the value for a timeuuid or timestamp column, you can give the value now, in which case CQErl will generate a normal timestamp, or a UUID (v1) matching the current date and time.

3. You can tell CQErl to consider a query reusable or not (see below for what that means). By default, it will detect binding variables and consider it reusable if it contains (named or not) any. Queries containing named binding variables will be considered reusable no matter what you set reusable to. If you explicitely set reusable to false on a query having positional variable bindings (?), you would provide values with in {Type, Value} pairs instead of {Key, Value}.

4. You can specify how many rows you want in every result page using the page_size (integer) field. The devs at Cassandra recommend a value of 100 (which is the default).

5. You can also specify what consistency you want the query to be executed under. Possible values include (all defined in include/cqerl.hrl):

   • ?CQERL_CONSISTENCY_ANY
   • ?CQERL_CONSISTENCY_ONE
   • ?CQERL_CONSISTENCY_TWO
   • ?CQERL_CONSISTENCY_THREE
   • ?CQERL_CONSISTENCY_QUORUM
   • ?CQERL_CONSISTENCY_ALL
   • ?CQERL_CONSISTENCY_LOCAL_QUORUM
   • ?CQERL_CONSISTENCY_EACH_QUORUM
   • ?CQERL_CONSISTENCY_LOCAL_ONE

6. In case you want to perform a lightweight transaction using INSERT or UPDATE, you can also specify the serial_consistency that will be use when performing it. Possible values are:

   • ?CQERL_CONSISTENCY_SERIAL
   • ?CQERL_CONSISTENCY_LOCAL_SERIAL

Batched queries

To perform batched queries (which can include any non-SELECT DML statements), simply put one or more #cql_query{} records in a #cql_query_batch{} record, and run it in place of a normal #cql_query{}. #cql_query_batch{} include the following fields:

1. The consistency level to apply when executing the batch of queries.
2. The mode of the batch, which can be ?CQERL_BATCH_LOGGED, ?CQERL_BATCH_UNLOGGED or ?CQERL_BATCH_COUNTER (declared in include/cqerl.hrl). Running a batch in unlogged mode removes the performance penalty of enforcing atomicity. The counter mode should be used to perform batched mutation of counter values.
3. Finally, you must specify the list of queries.

InsertQ = #cql_query{statement = "INSERT INTO users(id, name, password) VALUES(?, ?, ?);"},
{ok, void} = cqerl:run_query(Client, #cql_query_batch{
  mode=?CQERL_BATCH_UNLOGGED,
  queries=[
    InsertQ#cql_query{values = [{id, new},{name, "sean"},{password, "12312"}]},
    InsertQ#cql_query{values = [{id, new},{name, "jenna"},{password, "11111"}]},
    InsertQ#cql_query{values = [{id, new},{name, "kate"},{password, "foobar"}]}
  ]
}).

Reusable queries

If any of the following is true:

• you set #cql_query{}'s reusable field to true
• the query contains positional variable bindings (?) and you did not explicitely reusable to false
• the query contains named variable bindings (:name) (ignores the value of reusable)

the query is considered reusable. This means that the first time this query will be performed, CQErl will ask the connected Cassandra node to prepare the query, after which, internally, a query ID will be used instead of the query statement when executing it. That particular cassandra node will hold on to the prepared query on its side and subsequent queries that use exactly the same statement will be performed faster and with less network traffic.

CQErl can tell which query has been previously prepared on which node by keeping a local cache, so all of this happens correctly and transparently.

Data types

Here is a correspondance of cassandra column types with their equivalent Erlang types (bold denotes what will used in result sets, the rest is what is accepted).

Cassandra Column Type	Erlang types
ascii	binary, string (only US-ASCII)
bigint	integer (signed 64-bit)
blob	binary
boolean	true, false
counter	integer (signed 64-bit)
decimal	{Unscaled :: integer(), Scale :: integer()}
double	float (signed 64-bit)
float	float (signed 32-bit)
int	integer (signed 32-bit)
timestamp	integer (signed 64-bit), now, binary or string
uuid	binary, new
varchar	binary, string
varint	integer (arbitrary precision)
timeuuid	binary, now
inet	{X1, X2, X3, X4} (IPv4), {Y1, Y2, Y3, Y4, Y5, Y6, Y7, Y8} (IPv6), string or binary

Installation

Just include this repository in your project's rebar.config file and run ./rebar get-deps. See rebar for more details on how to use rebar for Erlang project management.

Tests

CQErl includes a test suite that you can run yourself, especially if you plan to contribute to this project.

1. Clone this repo on your machine
2. Edit test/test.config and put your own cassandra's configurations
3. At the project's top directory, run make test

License

The MIT License (MIT)

Copyright (c) 2013 Mathieu D'Amours

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.