Find file
Fetching contributors…
Cannot retrieve contributors at this time
372 lines (281 sloc) 11.9 KB
Riak Client Usage Introduction
This document assumes that you have already started your Riak cluster.
For instructions on that prerequisite, refer to
To talk to riak, all you need is an Erlang node with the Riak ebin
directories in its code path. Once this shell is up, use
riak:client_connect/1 to get connected. The client returned from
client_connect is defined by the riak_client module, and supports the
simple functions get, put, delete, and others.
Starting Your Client Erlang Node
Riak client nodes must use "long names" and have the Riak ebin
directoriess in their code path. The easiest way to start a node of
this nature is:
$ export ERL_LIBS=$PATH_TO_RIAK/apps
$ erl -name myclient@ -setcookie cookie
Note: If you are using a precompiled version of Riak your ERL_LIBS
would be /usr/lib/riak/lib
You'll know you've done this correctly if you can execute the
following commands and get a path to a beam file, instead of the atom
(myclient@> code:which(riak).
Once you have your node running, pass your Riak server nodename
to riak:client_connect/1 to connect and get a client. This can
be as simple as:
3> {ok, Client} = riak:client_connect('riak@').
{ok,{riak_client,'riak@', <<1,112,224,226>>}}
Storing New Data
Each bit of data in Riak is stored in a "bucket" at a "key" that is
unique to that bucket. The bucket is intended as an organizational
aid, for example to help segregate data by type, but Riak doesn't care
what values it stores, so choose whatever scheme suits you. Buckets
and keys must both be binaries.
Before storing your data, you must wrap it in a riak_object:
4> Object = riak_object:new(<<"groceries">>, <<"mine">>, ["eggs", "bacon"]).
Then, using the client you opened earlier, store the object:
5> Client:put(Object, 1).
If the return value of the last command was anything but the atom
'ok', then the store failed. The return value may give you a clue as
to why the store failed, but check the Troubleshooting section below if
The object is now stored in Riak, but you may be wondering about the
additional parameter to Client:put. There are five different 'put'
functions: put/1, put/2, put/3, put/4 and put/5. The lower-arity functions pass
defaults for the parameters they leave out of the higher-arity
functions. The available parameters, in order are:
Object: the riak_object to store
W: the minimum number of nodes that must respond
with success for the write to be considered successful
DW: the minimum number of nodes that must respond
with success *after durably storing* the object for the
write to be considered successful
Timeout: the number of milliseconds to wait for W and DW responses
before exiting with a timeout
Options: List of options that control certain behavior.
returnbody: Return the stored object.
The default timeout is currently 60 seconds, and put/2 passes its W
value as the DW value. So, the example above asks the client to store
Object, waiting for 1 successful durable write response, waiting a
maximum of 60 seconds for success.
See riak/doc/architecture.txt for more information about W and DW
Fetching Data
At some point you'll want that data back. Using the same bucket and
key you used before:
6> {ok, O} = Client:get(<<"groceries">>, <<"mine">>, 1).
7> riak_object:get_value(O).
Like 'put', there are multiple 'get' functions: get/2, get/3, and get/4. Their
parameters are:
Bucket: the bucket in which the object is stored
Key: the key under which the object is stored
R: the minimum number of nodes that must respond
with success for the read to be considered successful
Timeout: the number of milliseconds to wait for R responses
before exiting with a timeout
So, the example 'get' above requested the "mine" object in the
"groceries" bucket, demanding at least one successful response in 60
Modifying Data
Say you had the "grocery list" from the examples above, reminding you
to get ["eggs","bacon"], and you want to add "milk" to it. The
easiest way is:
8> {ok, Oa} = Client:get(<<"groceries">>, <<"mine">>, 1).
9> Ob = riak_object:update_value(Oa, ["milk"|riak_object:get_value(Oa)]).
10> Client:put(Ob).
That is, fetch the object from Riak, modify its value with
riak_object:update_value/2, then store the modified object back in
Riak. You can get your updated object to convince yourself that your
list is updated:
11> {ok, Oc} = Client:get(<<"groceries">>, <<"mine">>, 1).
12> riak_object:get_value(Oc).
By default, Riak does not expose siblings to clients. It is, however,
important to be aware of their presence.
Riak is able to provide high availability, in part, due to its ability
to accept put requests, even when it can't tell if that put would
overwrite data. Through the use of vector clocks Riak is able to track
and resolve conflicting versions of an object. For more information
about how Riak uses vclocks, see riak/doc/architecture.txt.
Siblings occur when Riak accepts conflicting values for an object that
cannot be resolved with vector clocks.
Let's continue the example from above but this time we will tell Riak to
return siblings to the client. Siblings can be allowed per bucket by
setting 'allow_mult' to 'true':
13> Client:set_bucket(<<"groceries">>, [{allow_mult, true}]).
To create a sibling, we'll fire up a new client and define our grocery
list with different items:
14> {ok, Client2} = riak:client_connect('riak@').
15> Client2:put(riak_object:new(<<"groceries">>, <<"mine">>, ["bread","cheese"]), 1).
16> {ok, O2} = Client2:get(<<"groceries">>, <<"mine">>, 1).
17> riak_object:get_value(O2).
** exception error: no match of right hand side value
in function riak_object:get_value/1
Now that Riak is returning siblings it seems our get_value/1 function
is broken. So, what happened? The function get_value/1 does not know
how to handle siblings; to see both sets of conflicting data, use
18> riak_object:get_values(O2).
It is up to the client to "merge" the siblings in whatever way suits
your application. In this case, we really did just want bread and
cheese in the list, so:
19> O3 = riak_object:update_value(O2, ["bread","cheese"]).
20> Client2:put(O3, 1).
Now, we can fetch the grocery list and check its value:
21> {ok, O4} = Client2:get(<<"groceries">>, <<"mine">>, 1).
22> riak_object:get_value(O4).
To return to the default behavior we can set 'allow_mult' back to false:
23> Client2:set_bucket(<<"groceries">>, [{allow_mult, false}]).
Listing Keys
Most uses of key-value stores are structured in such a way that
requests know which keys they want in a bucket. Sometimes, though,
it's necessary to find out what keys are available (when debugging,
for example). For that, there is list_keys:
1> Client:list_keys(<<"groceries">>).
{ok, ["mine"]}.
Note that keylist updates are asynchronous to the object storage
primitives, and may not be updated immediately after a put or delete.
This function is primarily intended as a debugging aid.
Deleting Data
Throwing away data is quick and simple: just use the delete function
in the riak_client module:
1> Client:delete(<<"groceries">>, <<"mine">>, 1).
As with get, delete has arity-2, arity-3 and arity-4 functions, with
Bucket: the bucket the object is in
Key: the key to delete
RW: the number of nodes to wait for responses from
Timeout: the number of milliseconds to wait for responses
So, the command demonstrated above tries to delete the object with the
key "mine" in the bucket "groceries", waiting up to 60 seconds for at
least one node to respond.
Issuing a delete for an object that does not exist returns an error
tuple. For example, calling the same delete as above a second time:
2> Client:delete(<<"groceries">>, <<"mine">>, 1).
Bucket Properties
As seen in the examples above, simply storing a key/value in a bucket
causes the bucket to come into existence with some default parameters.
To view the settings for a bucket, use the get_bucket function in the
riak_client module:
1> Client:get_bucket(<<"groceries">>).
If the default parameters do not suit your application, you can alter
them on a per-bucket basis with set_bucket. You should do this before
storing any data in the bucket, but many of the settings will "just
work" if they are modified after the bucket contains data.
An interesting bucket setting to take note of is 'n_val'. n_val tells
Riak how many copies of an object to make. More properly, for a bucket
with an n_val of N, Riak will store each object in the bucket in N
different vnodes (see riak/doc/architecture.txt for a description of
the term "vnode").
Most of the time, the default n_val of 3 works perfectly. Three
provides some durability and availability over 1 or 2.
If you are attempting to enhance either durability or speed, however,
increasing n_val may make sense *if you also modify the R, W, and DW*
values in your get and put calls. Changing n_val on its own will have
little effect without also modifying the put- and get-time parameters.
{nodedown, ...}
If all of your riak_client calls are exiting with exceptions that
describe themselves as, roughly:
** exception exit: {{nodedown,'riak@'},
in function gen_server:call/3
The node that your client was connected to is down. Try connecting a
new client.
riak:client_connect/1 returns {error,timeout}
The Riak node you are connecting to is down.
The bucket/key combination you requested was not found.