cron

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Published: May 24, 2019 License: MIT Imports: 9 Imported by: 0

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cron

DRAFT - Upgrading to v3

cron v3 is a major upgrade to the library that addresses all outstanding bugs, feature requests, and clarifications around usage. It is based on a merge of master which contains various fixes to issues found over the years and the v2 branch which contains some backwards-incompatible features like the ability to remove cron jobs. In addition, v3 adds support for Go Modules and cleans up rough edges like the timezone support.

It is currently IN DEVELOPMENT and will be considered released once a 3.0 version is tagged. It is backwards INCOMPATIBLE with both the v1 and v2 branches.

Updates required:

  • The v1 branch accepted an optional seconds field at the beginning of the cron spec. This is non-standard and has led to a lot of confusion. The new default parser conforms to the standard as described by the Cron wikipedia page.

    UPDATING: To retain the old behavior, construct your Cron with a custom parser:

    // Seconds field, required
    cron.New(cron.WithSeconds())
    
    // Seconds field, optional
    cron.New(
        cron.WithParser(
            cron.SecondOptional | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.Descriptor))
    
  • The Cron type now accepts functional options on construction rather than the ad-hoc behavior modification mechanisms before (setting a field, calling a setter).

    UPDATING: Code that sets Cron.ErrorLogger or calls Cron.SetLocation must be updated to provide those values on construction.

  • CRON_TZ is now the recommended way to specify the timezone of a single schedule, which is sanctioned by the specification. The legacy "TZ=" prefix will continue to be supported since it is unambiguous and easy to do so.

    UPDATING: No update is required.

Planned updates before calling v3 done:

  • Job "Interceptors" (name tbd), which make it easy for callers to mix desired behavior like the following:

    • Recover any panics from jobs
    • Block this job if the previous run hasn't completed yet
    • Logging job invocations
    • Notification when jobs are completed
  • Fix all open bugs

Background - Cron spec format

There are two cron spec formats in common usage:

The original version of this package included an optional "seconds" field, which made it incompatible with both of these formats. Instead, the schedule parser has been extended to support both types.

Documentation

Overview

Package cron implements a cron spec parser and job runner.

Usage

Callers may register Funcs to be invoked on a given schedule. Cron will run them in their own goroutines.

c := cron.New()
c.AddFunc("30 * * * *", func() { fmt.Println("Every hour on the half hour") })
c.AddFunc("30 3-6,20-23 * * *", func() { fmt.Println(".. in the range 3-6am, 8-11pm") })
c.AddFunc("CRON_TZ=Asia/Tokyo 30 04 * * * *", func() { fmt.Println("Runs at 04:30 Tokyo time every day") })
c.AddFunc("@hourly",      func() { fmt.Println("Every hour") })
c.AddFunc("@every 1h30m", func() { fmt.Println("Every hour thirty") })
c.Start()
..
// Funcs are invoked in their own goroutine, asynchronously.
...
// Funcs may also be added to a running Cron
c.AddFunc("@daily", func() { fmt.Println("Every day") })
..
// Inspect the cron job entries' next and previous run times.
inspect(c.Entries())
..
c.Stop()  // Stop the scheduler (does not stop any jobs already running).

CRON Expression Format

A cron expression represents a set of times, using 5 space-separated fields.

Field name   | Mandatory? | Allowed values  | Allowed special characters
----------   | ---------- | --------------  | --------------------------
Minutes      | Yes        | 0-59            | * / , -
Hours        | Yes        | 0-23            | * / , -
Day of month | Yes        | 1-31            | * / , - ?
Month        | Yes        | 1-12 or JAN-DEC | * / , -
Day of week  | Yes        | 0-6 or SUN-SAT  | * / , - ?

Month and Day-of-week field values are case insensitive. "SUN", "Sun", and "sun" are equally accepted.

The specific interpretation of the format is based on the Cron Wikipedia page: https://en.wikipedia.org/wiki/Cron

Alternative Formats

Alternative Cron expression formats support other fields like seconds. You can implement that by creating a custom Parser as follows.

cron.New(
    cron.WithParser(
        cron.SecondOptional | cron.Hour | cron.Dom | cron.Month | cron.Dow | cron.Descriptor))

The most popular alternative Cron expression format is Quartz: http://www.quartz-scheduler.org/documentation/quartz-2.x/tutorials/crontrigger.html

Special Characters

Asterisk ( * )

The asterisk indicates that the cron expression will match for all values of the field; e.g., using an asterisk in the 5th field (month) would indicate every month.

Slash ( / )

Slashes are used to describe increments of ranges. For example 3-59/15 in the 1st field (minutes) would indicate the 3rd minute of the hour and every 15 minutes thereafter. The form "*\/..." is equivalent to the form "first-last/...", that is, an increment over the largest possible range of the field. The form "N/..." is accepted as meaning "N-MAX/...", that is, starting at N, use the increment until the end of that specific range. It does not wrap around.

Comma ( , )

Commas are used to separate items of a list. For example, using "MON,WED,FRI" in the 5th field (day of week) would mean Mondays, Wednesdays and Fridays.

Hyphen ( - )

Hyphens are used to define ranges. For example, 9-17 would indicate every hour between 9am and 5pm inclusive.

Question mark ( ? )

Question mark may be used instead of '*' for leaving either day-of-month or day-of-week blank.

Predefined schedules

You may use one of several pre-defined schedules in place of a cron expression.

Entry                  | Description                                | Equivalent To
-----                  | -----------                                | -------------
@yearly (or @annually) | Run once a year, midnight, Jan. 1st        | 0 0 1 1 *
@monthly               | Run once a month, midnight, first of month | 0 0 1 * *
@weekly                | Run once a week, midnight between Sat/Sun  | 0 0 * * 0
@daily (or @midnight)  | Run once a day, midnight                   | 0 0 * * *
@hourly                | Run once an hour, beginning of hour        | 0 * * * *

Intervals

You may also schedule a job to execute at fixed intervals, starting at the time it's added or cron is run. This is supported by formatting the cron spec like this:

@every <duration>

where "duration" is a string accepted by time.ParseDuration (http://golang.org/pkg/time/#ParseDuration).

For example, "@every 1h30m10s" would indicate a schedule that activates after 1 hour, 30 minutes, 10 seconds, and then every interval after that.

Note: The interval does not take the job runtime into account. For example, if a job takes 3 minutes to run, and it is scheduled to run every 5 minutes, it will have only 2 minutes of idle time between each run.

Time zones

By default, all interpretation and scheduling is done in the machine's local time zone (time.Local). You can specify a different time zone on construction:

cron.New(
    cron.WithLocation(time.UTC))

Individual cron schedules may also override the time zone they are to be interpreted in by providing an additional space-separated field at the beginning of the cron spec, of the form "CRON_TZ=Asia/Tokyo".

For example:

# Runs at 6am in time.Local
cron.New().AddFunc("0 6 * * ?", ...)

# Runs at 6am in America/New_York
nyc, _ := time.LoadLocation("America/New_York")
c := cron.New(cron.WithLocation(nyc))
c.AddFunc("0 6 * * ?", ...)

# Runs at 6am in Asia/Tokyo
cron.New().AddFunc("CRON_TZ=Asia/Tokyo 0 6 * * ?", ...)

# Runs at 6am in Asia/Tokyo
c := cron.New(cron.WithLocation(nyc))
c.SetLocation("America/New_York")
c.AddFunc("CRON_TZ=Asia/Tokyo 0 6 * * ?", ...)

The prefix "TZ=(TIME ZONE)" is also supported for legacy compatibility.

Be aware that jobs scheduled during daylight-savings leap-ahead transitions will not be run!

Thread safety

Since the Cron service runs concurrently with the calling code, some amount of care must be taken to ensure proper synchronization.

All cron methods are designed to be correctly synchronized as long as the caller ensures that invocations have a clear happens-before ordering between them.

Implementation

Cron entries are stored in an array, sorted by their next activation time. Cron sleeps until the next job is due to be run.

Upon waking:

  • it runs each entry that is active on that second
  • it calculates the next run times for the jobs that were run
  • it re-sorts the array of entries by next activation time.
  • it goes to sleep until the soonest job.

Index

Constants

This section is empty.

Variables

This section is empty.

Functions

This section is empty.

Types

type ConstantDelaySchedule

type ConstantDelaySchedule struct {
	Delay time.Duration
}

ConstantDelaySchedule represents a simple recurring duty cycle, e.g. "Every 5 minutes". It does not support jobs more frequent than once a second.

func Every

func Every(duration time.Duration) ConstantDelaySchedule

Every returns a crontab Schedule that activates once every duration. Delays of less than a second are not supported (will round up to 1 second). Any fields less than a Second are truncated.

func (ConstantDelaySchedule) Next

func (schedule ConstantDelaySchedule) Next(t time.Time) time.Time

Next returns the next time this should be run. This rounds so that the next activation time will be on the second.

func (ConstantDelaySchedule) Prev

func (schedule ConstantDelaySchedule) Prev(t time.Time) time.Time

type Cron

type Cron struct {
	// contains filtered or unexported fields
}

Cron keeps track of any number of entries, invoking the associated func as specified by the schedule. It may be started, stopped, and the entries may be inspected while running.

func New

func New(opts ...Option) *Cron

New returns a new Cron job runner, modified by the given options.

Available Settings

Time Zone
  Description: The time zone in which schedules are interpreted
  Default:     time.Local

PanicLogger
  Description: How to log Jobs that panic
  Default:     Log the panic to os.Stderr

Parser
  Description:
  Default:     Parser that accepts the spec described here: https://en.wikipedia.org/wiki/Cron

See "cron.With*" to modify the default behavior.

func (*Cron) AddFunc

func (c *Cron) AddFunc(spec string, cmd func()) (EntryID, error)

AddFunc adds a func to the Cron to be run on the given schedule. The spec is parsed using the time zone of this Cron instance as the default. An opaque ID is returned that can be used to later remove it.

func (*Cron) AddJob

func (c *Cron) AddJob(spec string, cmd Job) (EntryID, error)

AddJob adds a Job to the Cron to be run on the given schedule. The spec is parsed using the time zone of this Cron instance as the default. An opaque ID is returned that can be used to later remove it.

func (*Cron) Entries

func (c *Cron) Entries() []Entry

Entries returns a snapshot of the cron entries.

func (*Cron) Entry

func (c *Cron) Entry(id EntryID) Entry

Entry returns a snapshot of the given entry, or nil if it couldn't be found.

func (*Cron) Location

func (c *Cron) Location() *time.Location

Location gets the time zone location

func (*Cron) Remove

func (c *Cron) Remove(id EntryID)

Remove an entry from being run in the future.

func (*Cron) Run

func (c *Cron) Run()

Run the cron scheduler, or no-op if already running.

func (*Cron) Schedule

func (c *Cron) Schedule(schedule Schedule, cmd Job) EntryID

Schedule adds a Job to the Cron to be run on the given schedule.

func (*Cron) Start

func (c *Cron) Start()

Start the cron scheduler in its own goroutine, or no-op if already started.

func (*Cron) Stop

func (c *Cron) Stop()

Stop stops the cron scheduler if it is running; otherwise it does nothing.

type Entry

type Entry struct {
	// ID is the cron-assigned ID of this entry, which may be used to look up a
	// snapshot or remove it.
	ID EntryID

	// Schedule on which this job should be run.
	Schedule Schedule

	// Next time the job will run, or the zero time if Cron has not been
	// started or this entry's schedule is unsatisfiable
	Next time.Time

	// Prev is the last time this job was run, or the zero time if never.
	Prev time.Time

	// Job is the thing to run when the Schedule is activated.
	Job Job
}

Entry consists of a schedule and the func to execute on that schedule.

func (Entry) Valid

func (e Entry) Valid() bool

Valid returns true if this is not the zero entry.

type EntryID

type EntryID int

EntryID identifies an entry within a Cron instance

type FuncJob

type FuncJob func()

FuncJob is a wrapper that turns a func() into a cron.Job

func (FuncJob) Run

func (f FuncJob) Run()

type Job

type Job interface {
	Run()
}

Job is an interface for submitted cron jobs.

type Option

type Option func(*Cron)

Option represents a modification to the default behavior of a Cron.

func WithLocation

func WithLocation(loc *time.Location) Option

WithLocation overrides the timezone of the cron instance.

func WithPanicLogger

func WithPanicLogger(l *log.Logger) Option

WithPanicLogger overrides the logger used for logging job panics.

func WithParser

func WithParser(p Parser) Option

WithParser overrides the parser used for interpreting job schedules.

func WithSeconds

func WithSeconds() Option

WithSeconds overrides the parser used for interpreting job schedules to include a seconds field as the first one.

type ParseOption

type ParseOption int

Configuration options for creating a parser. Most options specify which fields should be included, while others enable features. If a field is not included the parser will assume a default value. These options do not change the order fields are parse in.

const (
	Second         ParseOption = 1 << iota // Seconds field, default 0
	SecondOptional                         // Optional seconds field, default 0
	Minute                                 // Minutes field, default 0
	Hour                                   // Hours field, default 0
	Dom                                    // Day of month field, default *
	Month                                  // Month field, default *
	Dow                                    // Day of week field, default *
	DowOptional                            // Optional day of week field, default *
	Descriptor                             // Allow descriptors such as @monthly, @weekly, etc.
)

type Parser

type Parser struct {
	// contains filtered or unexported fields
}

A custom Parser that can be configured.

func NewParser

func NewParser(options ParseOption) Parser

NewParser creates a Parser with custom options.

It panics if more than one Optional is given, since it would be impossible to correctly infer which optional is provided or missing in general.

Examples

// Standard parser without descriptors
specParser := NewParser(Minute | Hour | Dom | Month | Dow)
sched, err := specParser.Parse("0 0 15 */3 *")

// Same as above, just excludes time fields
subsParser := NewParser(Dom | Month | Dow)
sched, err := specParser.Parse("15 */3 *")

// Same as above, just makes Dow optional
subsParser := NewParser(Dom | Month | DowOptional)
sched, err := specParser.Parse("15 */3")

func (Parser) Parse

func (p Parser) Parse(spec string) (Schedule, error)

Parse returns a new crontab schedule representing the given spec. It returns a descriptive error if the spec is not valid. It accepts crontab specs and features configured by NewParser.

type Schedule

type Schedule interface {
	// Next returns the next activation time, later than the given time.
	// Next is invoked initially, and then each time the job is run.
	Next(time.Time) time.Time

	// Return the previous activation time, before than the given time.
	Prev(time.Time) time.Time
}

Schedule describes a job's duty cycle.

func ParseStandard

func ParseStandard(standardSpec string) (Schedule, error)

ParseStandard returns a new crontab schedule representing the given standardSpec (https://en.wikipedia.org/wiki/Cron). It differs from Parse requiring to always pass 5 entries representing: minute, hour, day of month, month and day of week, in that order. It returns a descriptive error if the spec is not valid.

It accepts

  • Standard crontab specs, e.g. "* * * * ?"
  • Descriptors, e.g. "@midnight", "@every 1h30m"

type SpecSchedule

type SpecSchedule struct {
	Second, Minute, Hour, Dom, Month, Dow uint64

	// Override location for this schedule.
	Location *time.Location
}

SpecSchedule specifies a duty cycle (to the second granularity), based on a traditional crontab specification. It is computed initially and stored as bit sets.

func (*SpecSchedule) Next

func (s *SpecSchedule) Next(t time.Time) time.Time

Next returns the next time this schedule is activated, greater than the given time. If no time can be found to satisfy the schedule, return the zero time.

func (*SpecSchedule) Prev

func (s *SpecSchedule) Prev(t time.Time) time.Time

Prev returns the previous time this schedule is activated, lesser than the given time. If no time can be found to satisfy the schedule, return the zero time.

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