mesh-sdk/go/process/follow.go

package process

import (
	"encoding/json"
	"fmt"
	"time"

	"github.com/nats-io/nats.go"

	"git.novox.be/novox/mesh-sdk/go/membership"
)

// FollowMembership reads this account's own membership and follows it (novox/hq ADR 0160): apply is called
// with every membership the mesh issues this account's node and module, the one already issued first and
// each later one as it is published, so a change reaches a running process without a restart. A body that is
// not a membership is handed to onBad (nil to ignore it) and skipped.
//
// The subject is followed before it is read, so an issue landing between the two is applied, not missed. The
// read is the subject-addressed direct get, the one request this account may make of the stream; no
// membership issued yet, or the read failing, is not an error: the process follows the subject and is told
// when one arrives. The returned error is a refusal to follow at all.
func (c *Conn) FollowMembership(apply func(membership.Membership), onBad func(error)) error {
	if c.account.Node == "" || c.account.Module == "" {
		return fmt.Errorf("the account names no node or module, so it has no membership")
	}
	bad := func(err error) {
		if onBad != nil {
			onBad(err)
		}
	}
	take := func(body []byte) {
		var issued membership.Membership
		if err := json.Unmarshal(body, &issued); err != nil {
			bad(fmt.Errorf("a membership arrived that is not one: %w", err))
			return
		}
		apply(issued)
	}
	subject := membership.Subject(c.account.Node, c.account.Module)
	if _, err := c.nc.Subscribe(subject, func(m *nats.Msg) { take(m.Data) }); err != nil {
		return fmt.Errorf("cannot follow %s: %w", subject, err)
	}
	got, err := c.nc.Request(membership.DirectGetSubject(c.account.Node, c.account.Module), nil, 5*time.Second)
	switch {
	case err != nil:
		bad(fmt.Errorf("cannot read the membership issued on %s yet (%v); following it", subject, err))
	case got.Header.Get("Status") != "" || len(got.Data) == 0:
		// None issued yet: the follow above delivers it.
	default:
		take(got.Data)
	}
	return nil
}