Tracing
Tracing is a ScyllaDB tool that is meant to help debugging and analysing the internal flows in the server. One example of such a flow is a CQL request processing.
If you are going to use either a probabilistic tracing or a Slow Query Logging (see below) then it's strongly recommended to change the replication factor of the system_traces KS to ALL (a total number of Nodes in your cluster).
Users of a cqlsh may enable Tracing with TRACING ON|OFF command:
cqlsh> TRACING ON
Now Tracing is enabled
cqlsh> INSERT into keyspace1.standard1 (key, "C0") VALUES (0x12345679, bigintAsBlob(123456));
Tracing session: 227aff60-4f21-11e6-8835-000000000000
activity | timestamp | source | source_elapsed
--------------------------------------------------------------------------------------------------+----------------------------+-----------+----------------
Execute CQL3 query | 2016-07-21 11:57:21.238000 | 127.0.0.2 | 0
Parsing a statement [shard 1] | 2016-07-21 11:57:21.238335 | 127.0.0.2 | 1
Processing a statement [shard 1] | 2016-07-21 11:57:21.238405 | 127.0.0.2 | 71
Creating write handler for token: 2309717968349690594 natural: {127.0.0.1} pending: {} [shard 1] | 2016-07-21 11:57:21.238433 | 127.0.0.2 | 99
Creating write handler with live: {127.0.0.1} dead: {} [shard 1] | 2016-07-21 11:57:21.238439 | 127.0.0.2 | 105
Sending a mutation to /127.0.0.1 [shard 1] | 2016-07-21 11:57:21.238490 | 127.0.0.2 | 156
Message received from /127.0.0.2 [shard 0] | 2016-07-21 11:57:21.238562 | 127.0.0.1 | 17
Sending mutation_done to /127.0.0.2 [shard 0] | 2016-07-21 11:57:21.238658 | 127.0.0.1 | 113
Mutation handling is done [shard 0] | 2016-07-21 11:57:21.238675 | 127.0.0.1 | 130
Got a response from /127.0.0.1 [shard 1] | 2016-07-21 11:57:21.238950 | 127.0.0.2 | 616
Mutation successfully completed [shard 1] | 2016-07-21 11:57:21.238958 | 127.0.0.2 | 624
Done processing - preparing a result [shard 1] | 2016-07-21 11:57:21.238962 | 127.0.0.2 | 628
Request complete | 2016-07-21 11:57:21.238639 | 127.0.0.2 | 639
Note how source_elapsed starts over on 127.0.0.1 when execution gets there.
The raw tracing data looks as follows:
cqlsh> select * from system_traces.sessions where session_id=227aff60-4f21-11e6-8835-000000000000;
session_id | client | command | coordinator | duration | parameters | request | started_at
--------------------------------------+-----------+---------+-------------+----------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+--------------------+---------------------------------
227aff60-4f21-11e6-8835-000000000000 | 127.0.0.1 | QUERY | 127.0.0.2 | 639 | {'consistency_level': 'ONE', 'page_size': '100', 'query': 'INSERT into keyspace1.standard1 (key, "C0") VALUES (0x12345679, bigintAsBlob(123456));', 'serial_consistency_level': 'SERIAL', 'user_timestamp': '1469091441238107'} | Execute CQL3 query | 2016-07-21 08:57:21.238000+0000
(1 rows)
cqlsh> select * from system_traces.events where session_id=227aff60-4f21-11e6-8835-000000000000;
session_id | event_id | activity | source | source_elapsed | thread
--------------------------------------+--------------------------------------+----------------------------------------------------------------------------------------+-----------+----------------+--------
227aff60-4f21-11e6-8835-000000000000 | 227b0c74-4f21-11e6-8835-000000000000 | Parsing a statement | 127.0.0.2 | 1 | shard 1
227aff60-4f21-11e6-8835-000000000000 | 227b0f34-4f21-11e6-8835-000000000000 | Processing a statement | 127.0.0.2 | 71 | shard 1
227aff60-4f21-11e6-8835-000000000000 | 227b1047-4f21-11e6-8835-000000000000 | Creating write handler for token: 2309717968349690594 natural: {127.0.0.1} pending: {} | 127.0.0.2 | 99 | shard 1
227aff60-4f21-11e6-8835-000000000000 | 227b1087-4f21-11e6-8835-000000000000 | Creating write handler with live: {127.0.0.1} dead: {} | 127.0.0.2 | 105 | shard 1
227aff60-4f21-11e6-8835-000000000000 | 227b1284-4f21-11e6-8835-000000000000 | Sending a mutation to /127.0.0.1 | 127.0.0.2 | 156 | shard 1
227aff60-4f21-11e6-8835-000000000000 | 227b1559-4f21-11e6-bf08-000000000000 | Message received from /127.0.0.2 | 127.0.0.1 | 17 | shard 0
227aff60-4f21-11e6-8835-000000000000 | 227b1915-4f21-11e6-bf08-000000000000 | Sending mutation_done to /127.0.0.2 | 127.0.0.1 | 113 | shard 0
227aff60-4f21-11e6-8835-000000000000 | 227b19bd-4f21-11e6-bf08-000000000000 | Mutation handling is done | 127.0.0.1 | 130 | shard 0
227aff60-4f21-11e6-8835-000000000000 | 227b247e-4f21-11e6-8835-000000000000 | Got a response from /127.0.0.1 | 127.0.0.2 | 616 | shard 1
227aff60-4f21-11e6-8835-000000000000 | 227b24ca-4f21-11e6-8835-000000000000 | Mutation successfully completed | 127.0.0.2 | 624 | shard 1
227aff60-4f21-11e6-8835-000000000000 | 227b24f2-4f21-11e6-8835-000000000000 | Done processing - preparing a result | 127.0.0.2 | 628 | shard 1
(11 rows)
cqlsh>
Tracing implies a significant performance penalty on a cluster when it's enabled. Therefore if we want to enable tracing for some on going workload we don't want to enable it for every request but rather for some (small) portion of them. This may be achieved using the so called probabilistic tracing, which would randomly choose a request to be traced with some defined probability.
For instance, if we want to trace 0.01% or all queries in the cluster we shell set a probabilistic tracing with the probability 0.0001:
$ nodetool settraceprobability 0.0001
Traces are stored in a system_traces keyspace for 24 hours, which consists of 2 tables with replication factor of 2:
CREATE TABLE system_traces.events (
session_id uuid,
event_id timeuuid,
activity text,
source inet,
source_elapsed int,
thread text,
PRIMARY KEY (session_id, event_id)
)
CREATE TABLE system_traces.sessions (
session_id uuid PRIMARY KEY,
client inet,
command text,
coordinator inet,
duration int,
parameters map<text, text>,
request text,
started_at timestamp
)
Traces are created in a context of a tracing session. For instance, if we trace an INSERT CQL command, a tracing session with a unique ID (session_id column in the tables above) will be created and all trace points hit during the execution will be stored in a context of this session. And this defines the format in which tracing data is stored:
-
sessionstable contains a single row for each tracing session -
eventstable contains a single row for each trace point.
If we need trace points for a specific session we may query events table for this session's ID (see examples above).
events columns are quite straight forward:
-
session_id: ID of a session this trace -
event_id: ID of this specific trace entry -
activity: a trace message -
source: address of a Node where the trace entry has been created -
source_elapsed: a number of microseconds passed since the beginning of the tracing session on a specific Node (see examples above) -
thread: currently this contains a number of a shard on which this trace point has been taken
-
session_id: ID of this tracing session -
command: currently this may only have a "QUERY" value -
client: address of a Client that has sent this query -
coordinator: address of a coordinator that received this query from a Client -
duration: the total duration of this tracing session -
parameters: this map contains string pairs that describe the query which may include:- query string
- consistency level
- etc.
-
request: a short string describing the current query, like "Execute CQL3 query" -
started_at: is a timestamp taken when tracing session has began
Many times in real life installations one of the most important parameters of the system is the longest response time. Naturally, the shorter it is - the better. Therefore capturing the request that take a long time and understanding why it took it so long is a very critical and challenging task.
The "slow query logging" is a ScyllaDB feature that is going to greatly ease the debugging related to the long requests.
When enabled it would record the queries with the handling time above the specified threshold. As a result there will be created a new record in a system_traces.node_slow_log table. All tracing records created in a context of this query on a Coordinator Node will be written as well. In addition, if handling on some replica takes too long, its traces are going to be stored too.
Thereby, when we detect a slow query we are going to get a valuable tracing data that would greatly help us to understand what was the query and why it took so long to complete.
By default slow query logging is disabled.
There is a REST API that allows configuring and querying the current configuration of this feature.
To set the parameters run:
curl -X POST --header "Content-Type: application/json" --header "Accept: application/json" "http://<Node's address>:10000/storage_service/slow_query?enable=<true|false>&ttl=<in seconds>&threshold=<threshold in microseconds>"
e.g. in order to disable the feature on a Node with an address 127.0.0.1, set the ttl to 8600s and a threshold to 10000us:
curl -X POST --header "Content-Type: application/json" --header "Accept: application/json" "http://127.0.0.1:10000/storage_service/slow_query?enable=false&ttl=8600&threshold=10000"
To get the current configuration run:
curl -X GET --header "Content-Type: application/json" --header "Accept: application/json" "http://<Node's address>:10000/storage_service/slow_query"
e.g. after the POST command above the query result will look as below:
$ curl -X GET --header "Content-Type: application/json" --header "Accept: application/json" "http://127.0.0.1:10000/storage_service/slow_query"
{"threshold": 10000, "enable": false, "ttl": 8600}
$
CREATE TABLE system_traces.node_slow_log (
start_time timeuuid,
node_ip inet,
shard int,
command text,
date timestamp,
duration int,
parameters map<text, text>,
session_id uuid,
source_ip inet,
table_names set<text>,
username text,
PRIMARY KEY (start_time, node_ip, shard)
)
-
start_timeanddate: time when the query has began -
node_ip: Address of a Coordinator Node -
shard: shard ID on a Coordinator, where the query has been handled -
command: the query command, e.g.select * from my_ks.my_cf -
duration: the duration of a query handling in miscroseconds -
parameters: query parameters like aparameterscolumn in a system_traces.sessions table -
session_id: the corresponding Tracing session ID -
source_ip: Address of a Client that sent this query -
table_names: a list of tables used for this query, where applicable -
username: a user name used for authentication with this query