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package jetstream
import (
"context"
"fmt"
"github.com/nats-io/nats.go"
"github.com/sirupsen/logrus"
)
func JetStreamConsumer(
ctx context.Context, js nats.JetStreamContext, subj, durable string,
f func(ctx context.Context, msg *nats.Msg) bool,
opts ...nats.SubOpt,
) error {
defer func() {
// If there are existing consumers from before they were pull
// consumers, we need to clean up the old push consumers. However,
// in order to not affect the interest-based policies, we need to
// do this *after* creating the new pull consumers, which have
// "Pull" suffixed to their name.
if _, err := js.ConsumerInfo(subj, durable); err == nil {
if err := js.DeleteConsumer(subj, durable); err != nil {
logrus.WithContext(ctx).Warnf("Failed to clean up old consumer %q", durable)
}
}
}()
name := durable + "Pull"
sub, err := js.PullSubscribe(subj, name, opts...)
if err != nil {
return fmt.Errorf("nats.SubscribeSync: %w", err)
}
go func() {
for {
// The context behaviour here is surprising — we supply a context
// so that we can interrupt the fetch if we want, but NATS will still
// enforce its own deadline (roughly 5 seconds by default). Therefore
// it is our responsibility to check whether our context expired or
// not when a context error is returned. Footguns. Footguns everywhere.
msgs, err := sub.Fetch(1, nats.Context(ctx))
if err != nil {
if err == context.Canceled || err == context.DeadlineExceeded {
// Work out whether it was the JetStream context that expired
// or whether it was our supplied context.
select {
case <-ctx.Done():
// The supplied context expired, so we want to stop the
// consumer altogether.
return
default:
// The JetStream context expired, so the fetch probably
// just timed out and we should try again.
continue
}
} else {
// Something else went wrong, so we'll panic.
logrus.WithContext(ctx).WithField("subject", subj).Fatal(err)
}
}
if len(msgs) < 1 {
continue
}
msg := msgs[0]
if err = msg.InProgress(); err != nil {
logrus.WithContext(ctx).WithField("subject", subj).Warn(fmt.Errorf("msg.InProgress: %w", err))
continue
}
if f(ctx, msg) {
if err = msg.Ack(); err != nil {
logrus.WithContext(ctx).WithField("subject", subj).Warn(fmt.Errorf("msg.Ack: %w", err))
}
} else {
if err = msg.Nak(); err != nil {
logrus.WithContext(ctx).WithField("subject", subj).Warn(fmt.Errorf("msg.Nak: %w", err))
}
}
}
}()
return nil
}
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