Scylla Summit 2017: How to Use Gocql to Execute Queries and What the Driver Does That You Can’t See

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HOW TO USE GOCQL TO
EXECUTE QUERIES AND WHAT
THE DRIVER DOES THAT YOU
CAN’T SEE
Chris Bannister

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CHRIS BANNISTER
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Software Engineer by day, Cassandra driver
developer at night. Been programming in Go for
over 4 years and working on GoCQL for 3.
Know far too much about Cassandra drivers.

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OUTLINE
▪ What is GoCQL
▪ Connecting to a cluster
▪ Querying
▪ Async querying
▪ Other libraries
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WHY USE GO
▪ Simple concurrency primitives
▪ First class functions
▪ Strongly typed
▪ Garbage collected
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WHAT IS GOCQL
▪ GoCQL is a driver for Cassandra & Scylla written in Go
▪ Supports all versions of Cassandra >= 2.0
▪ Supports nearly all the features of the Java driver
▪ Idiomatic Go API
▪ Has rich supporting libraries with more features
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INSTALLING
▪ go get github.com/gocql/gocql
▪ Docs at https://godoc.org/github.com/gocql/gocql
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CONNECTING TO A CLUSTER
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conf := gocql.NewCluster(“10.0.12.83“, “10.0.13.04”, “10.0.14.12”)
conf.PoolConfig.HostSelectionPolicy =
gocql.TokenAwareHostPolicy(gocql.RoundRobinHostPolicy())
conf.Compressor = &gocql.SnappyCompressor{}
conf.RetryPolicy = &gocql.ExponentialBackoffRetryPolicy{NumRetries: 3}
conf.Consistency = gocql.LocalQuorum
db, err := conf.CreateSession()
if err != nil {
// unable to connect
}

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if err != nil {
}

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if err != nil {
}

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if err != nil {
}

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conf := gocql.NewCluster(“10.0.12.83“, “10.0.13.04”, “10.0.14, 12”)
if err != nil {
}

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▪ Good defaults for production config
o Tailor to your environment
▪ Use a single session per application
o Session is like a connection pool, with extra features
▪ Supplied hosts only used to discover rest of the cluster
o Ensure cluster IP addresses are configured correctly!
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WHAT HAPPENED BEHIND THE SCENES
▪ Discover protocol version
o Pretty hacky, Cassandra please fix
▪ Discover cluster topology
o Addresses advertised by cluster take priority over those supplied
▪ Create connection pools to all discovered hosts
▪ Register for host and schema change events
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QUERYING AND ITERATING
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type TraceSpan struct {
ID gocql.UUID `cql:"id"`
StartTime time.Time `cql:"start_time"`
Duration time.Duration `cql:"duration"`
}
query := db.Query("SELECT span FROM traces WHERE service_name=?",
"my-service")
sc := query().Iter().Scanner()

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}
"my-service")
sc := query.Iter().Scanner()

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}
"my-service")
sc := query.Iter().Scanner()

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var spans []TraceSpan
for sc.Next() {
var span TraceSpan
if err := sc.Scan(&span); err != nil {
return err
}
spans = append(spans, span)
}
if err := sc.Err(); err != nil {
return err
}

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var spans []TraceSpan
for sc.Next() {
var span TraceSpan
return err
}
spans = append(spans, span)
}
return err
}

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▪ The driver automatically prefetches and pages results
▪ Check the error at the end
▪ Close iterators to let them be reused for performance
▪ Query options can be set per query
o Will inherit global settings from the session
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HOW QUERYING WORKS
▪ Pick host + Connection
o Token Aware, pick host which owns the data or a replica
o Connections can support up to 32k concurrent queries
▪ Prepare query
o Cached per host
o Required to know how to serialise the query parameters
▪ Serialise all outgoing values
▪ Deserialise results & return to caller
▪ Paginate, handle host errors
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ASYNC QUERIES IN GO
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ASYNC QUERIES IN GO
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ch := make(chan TraceSpan, 10)
go func() {
for sc.Next() {
var span TraceSpan
// handle error
}
ch <- span
}
// handle error
}
}()

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ASYNC QUERIES IN GO
▪ Gocql only provides a synchronous interface
▪ Trivial to adapt to be asynchronous
o Channels and Goroutines!
o Take care with errors
▪ Under the hood, all queries execute concurrently
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LIBRARIES USING GOCQL
▪ Gocql provides a low level and fully usable interface to Cassandra
▪ Other features available outside of the driver
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LIBRARIES USING GOCQL
▪ gocqlx is a Gocql extension that automates data binding, adds
named queries support, provides flexible query builders
o Thanks Scylla!
▪ gocassa provides query building, adds data binding, and provides
easy-to-use "recipe" tables for common query use-cases
▪ journey is a migration tool with Cassandra support
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THANK YOU
c.bannister@gmail.com
@ChrisBannister
Stay in touch!
Questions?

Scylla Summit 2017: How to Use Gocql to Execute Queries and What the Driver Does That You Can’t See

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Scylla Summit 2017: How to Use Gocql to Execute Queries and What the Driver Does That You Can’t See