I connect to the database in the init function of a controller, like:
db, err = sql.Open("mysql", "user:pass#tcp(<ip>:3306)/<db>")
if err != nil {
log.Fatal(err)
}
err = db.Ping()
if err != nil {
log.Fatal(err)
}
Then I prepare some statements (db.Prepare) and finally execute them somewhere in the code, without creating new db connections or anything weird. Just letting go handle the connection pool.
But as you can see in the image, I'm getting a lot of connections and aborted connections which make the server run slow and even crash.
Why is it happening? Also I have around 2000 simultaneous online users which result in about 20 queries per second. I don't think it's much, just in case it mattered.
EDIT:
Here's how I run the prepared statements. I have 2 selects, 1 update and 1 insert. Selects are being run like:
err = getStatement.QueryRow(apiKey).Scan(&keyId)
if err != nil {
res, _ := json.Marshal(RespError{"Statement Error"})
w.Write(res)
return
}
Inserts and updates:
insertStatement.Exec(a,b,c)
Related
I am developing a system to enable patient registration with incremental queue number. I am using Go, GORM, and MySQL.
An issue happens when more than one patients are registering at the same time, they tend to get the same queue number which it should not happen.
I attempted using transactions and hooks to achieve that but I still got duplicate queue number. I have not found any resource about how to lock the database when a transaction is happening.
func (r repository) CreatePatient(pat *model.Patient) error {
tx := r.db.Begin()
defer func() {
if r := recover(); r != nil {
tx.Rollback()
}
}()
err := tx.Error
if err != nil {
return err
}
// 1. get latest queue number and assign it to patient object
var queueNum int64
err = tx.Model(&model.Patient{}).Where("registration_id", pat.RegistrationID).Select("queue_number").Order("created_at desc").First(&queueNum).Error
if err != nil && err != gorm.ErrRecordNotFound {
tx.Rollback()
return err
}
pat.QueueNumber = queueNum + 1
// 2. write patient data into the db
err = tx.Create(pat).Error
if err != nil {
tx.Rollback()
return err
}
return tx.Commit().Error
}
As stated by #O. Jones, transactions don't save you here because you're extracting the largest value of a column, incrementing it outside the db and then saving that new value. From the database's point of view the updated value has no dependence on the queried value.
You could try doing the update in a single query, which would make the dependence obvious:
UPDATE patient AS p
JOIN (
SELECT max(queue_number) AS queue_number FROM patient WHERE registration_id = ?
) maxp
SET p.queue_number = maxp.queue_number + 1
WHERE id = ?
In gorm you can't run a complex update like this, so you'll need to make use of Exec.
I'm not 100% certain the above will work because I'm less familiar with MySQL transaction isolation guarantees.
A cleaner way
Overall, it'd be cleaner to keep a table of queues (by reference_id) with a counter that you update atomically:
Start a transaction, then
SELECT queue_number FROM queues WHERE registration_id = ? FOR UPDATE;
Increment the queue number in your app code, then
UPDATE queues SET queue_number = ? WHERE registration_id = ?;
Now you can use the incremented queue number in your patient creation/update before transaction commit.
The problem
I wrote an application which synchronizes data from BigQuery into a MySQL database. I try to insert roughly 10-20k rows in batches (up to 10 items each batch) every 3 hours. For some reason I receive the following error when it tries to upsert these rows into MySQL:
Can't create more than max_prepared_stmt_count statements:
Error 1461: Can't create more than max_prepared_stmt_count statements
(current value: 2000)
My "relevant code"
// ProcessProjectSkuCost receives the given sku cost entries and sends them in batches to upsertProjectSkuCosts()
func ProcessProjectSkuCost(done <-chan bigquery.SkuCost) {
var skuCosts []bigquery.SkuCost
var rowsAffected int64
for skuCostRow := range done {
skuCosts = append(skuCosts, skuCostRow)
if len(skuCosts) == 10 {
rowsAffected += upsertProjectSkuCosts(skuCosts)
skuCosts = []bigquery.SkuCost{}
}
}
if len(skuCosts) > 0 {
rowsAffected += upsertProjectSkuCosts(skuCosts)
}
log.Infof("Completed upserting project sku costs. Affected rows: '%d'", rowsAffected)
}
// upsertProjectSkuCosts inserts or updates ProjectSkuCosts into SQL in batches
func upsertProjectSkuCosts(skuCosts []bigquery.SkuCost) int64 {
// properties are table fields
tableFields := []string{"project_name", "sku_id", "sku_description", "usage_start_time", "usage_end_time",
"cost", "currency", "usage_amount", "usage_unit", "usage_amount_in_pricing_units", "usage_pricing_unit",
"invoice_month"}
tableFieldString := fmt.Sprintf("(%s)", strings.Join(tableFields, ","))
// placeholderstring for all to be inserted values
placeholderString := createPlaceholderString(tableFields)
valuePlaceholderString := ""
values := []interface{}{}
for _, row := range skuCosts {
valuePlaceholderString += fmt.Sprintf("(%s),", placeholderString)
values = append(values, row.ProjectName, row.SkuID, row.SkuDescription, row.UsageStartTime,
row.UsageEndTime, row.Cost, row.Currency, row.UsageAmount, row.UsageUnit,
row.UsageAmountInPricingUnits, row.UsagePricingUnit, row.InvoiceMonth)
}
valuePlaceholderString = strings.TrimSuffix(valuePlaceholderString, ",")
// put together SQL string
sqlString := fmt.Sprintf(`INSERT INTO
project_sku_cost %s VALUES %s ON DUPLICATE KEY UPDATE invoice_month=invoice_month`, tableFieldString, valuePlaceholderString)
sqlString = strings.TrimSpace(sqlString)
stmt, err := db.Prepare(sqlString)
if err != nil {
log.Warn("Error while preparing SQL statement to upsert project sku costs. ", err)
return 0
}
// execute query
res, err := stmt.Exec(values...)
if err != nil {
log.Warn("Error while executing statement to upsert project sku costs. ", err)
return 0
}
rowsAffected, err := res.RowsAffected()
if err != nil {
log.Warn("Error while trying to access affected rows ", err)
return 0
}
return rowsAffected
}
// createPlaceholderString creates a string which will be used for prepare statement (output looks like "(?,?,?)")
func createPlaceholderString(tableFields []string) string {
placeHolderString := ""
for range tableFields {
placeHolderString += "?,"
}
placeHolderString = strings.TrimSuffix(placeHolderString, ",")
return placeHolderString
}
My question:
Why do I hit the max_prepared_stmt_count when I immediately execute the prepared statement (see function upsertProjectSkuCosts)?
I could only imagine it's some sort of concurrency which creates tons of prepared statements in the meantime between preparing and executing all these statements. On the other hand I don't understand why there would be so much concurrency as the channel in the ProcessProjectSkuCost is a buffered channel with a size of 20.
You need to close the statement inside upsertProjectSkuCosts() (or re-use it - see the end of this post).
When you call db.Prepare(), a connection is taken from the internal connection pool (or a new connection is created, if there aren't any free connections). The statement is then prepared on that connection (if that connection isn't free when stmt.Exec() is called, the statement is then also prepared on another connection).
So this creates a statement inside your database for that connection. This statement will not magically disappear - having multiple prepared statements in a connection is perfectly valid. Golang could see that stmt goes out of scope, see it requires some sort of cleanup and then do that cleanup, but Golang doesn't (just like it doesn't close files for you and things like that). So you'll need to do that yourself using stmt.Close(). When you call stmt.Close(), the driver will send a command to the database server, telling it the statement is no longer needed.
The easiest way to do this is by adding defer stmt.Close() after the err check following db.Prepare().
What you can also do, is prepare the statement once and make that available for upsertProjectSkuCosts (either by passing the stmt into upsertProjectSkuCosts or by making upsertProjectSkuCosts a func of a struct, so the struct can have a property for the stmt). If you do this, you should not call stmt.Close() - because you aren't creating new statements anymore, you are re-using an existing statement.
Also see Should we also close DB's .Prepare() in Golang? and https://groups.google.com/forum/#!topic/golang-nuts/ISh22XXze-s
I have a problem with getting database table list (SHOW TABLES) in Go.
I use this packages
database/sql
gopkg.in/gorp.v1
github.com/ziutek/mymysql/godrv
and connect to MYSQL by this code:
db, err := sql.Open(
"mymysql",
"tcp:127.0.0.1:3306*test/root/root")
if err != nil {
panic(err)
}
dbmap := &DbMap{Conn:&gorp.DbMap{Db: db}}
And I use this code to get list of tables
result, _ := dbmap.Exec("SHOW TABLES")
But result is empty!
I use classic go-sql-driver/mysql:
db, _ := sql.Open("mysql", "root:qwerty#/dbname")
res, _ := db.Query("SHOW TABLES")
var table string
for res.Next() {
res.Scan(&table)
fmt.Println(table)
}
PS don't ignore errors! This is only an example
I'm trying this code and work successfully. I create a list of string and use Select query to get list of database tables.
tables := []string{}
dbmap.Select(&tables, "SHOW TABLES")
fmt.Println(tables)
I have an issue on Linux Ubuntu with 1.4.2, which I am not sure how to sort:
func main() {
dir, _ := filepath.Abs(filepath.Dir(os.Args[0]))
outputFile, outputError := os.OpenFile(dir+"/out1.csv",
os.O_WRONLY|os.O_CREATE, 0666)
if outputError != nil {
fmt.Printf("An error occurred with file creation\n")
return
}
defer outputFile.Close()
writer := csv.NewWriter(outputFile)
results := getResults()
for _, result := range results {
writer.Write([]string{result.Item, result.Price, result.Shipping})
}
writer.Flush()
}
when results is 1000+ records my PC freezes for seconds and when it's say 20k, it freezes for minutes.
How do I solve such an issue in a proper way?
I though to flush it every N records, and add time.Sleep – but that looks awkward…
Using the golang example below, how can I query (JOIN) multiple databases.
For example, I want to have the relation db1.username.id = db2.comments.username_id.
id := 123
var username string
err := db.QueryRow("SELECT username FROM users WHERE id=?", id).Scan(&username)
switch {
case err == sql.ErrNoRows:
log.Printf("No user with that ID.")
case err != nil:
log.Fatal(err)
default:
fmt.Printf("Username is %s\n", username)
}
As you are using MySQL, you can select fields across databases. See this related question for
details. For example you should be able to do this:
err := db.QueryRow(`
SELECT
db1.users.username
FROM
db1.users
JOIN
db2.comments
ON db1.users.id = db2.comments.username_id
`).Scan(&username)
You can of course simply fetch all entries from db2.comments using a second database connection and use the values in a query to db1.users. This is, of course, not recommended as it is the job of the database server which it can, most likely, do better than you.