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Varun-SethuVarun-Sethu
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feat(cms): Adding a projects-wide event service (#296)
* Added events service scaffold * updated environment variable * update codeowners * review changes * review changes
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‎CODEOWNERS‎

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backend/ @Varun-Sethu
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postgres/ @Varun-Sethu
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utilities/ @Varun-Sethu
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services/ @Varun-Sethu
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frontend/ @fafnirZ
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next/ @fafnirZ

‎services/events-service/Dockerfile‎

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FROM golang:1.19-alpine as app-builder
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WORKDIR /go/src/app
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COPY . .
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RUN apk add git
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# Static build required so that we can safely copy the binary over.
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# `-tags timetzdata` embeds zone info from the "time/tzdata" package.
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RUN CGO_ENABLED=0 go install -ldflags '-extldflags "-static"' -tags timetzdata
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FROM scratch
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# the test program:
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COPY --from=app-builder /go/bin/events-service /events-service
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# the tls certificates:
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# NB: this pulls directly from the upstream image, which already has ca-certificates:
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COPY --from=alpine:latest /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
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ENTRYPOINT ["/events-service"]

‎services/events-service/docker-compose.yml‎

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services:
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events-service:
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container_name: events-service
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build:
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context: .
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dockerfile: ./Dockerfile
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ports:
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- 8080:8080
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environment:
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- FB_TOKEN=please_replace_me

‎services/events-service/fetcher.go‎

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package main
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// Mostly copied from: https://github.com/csesoc/csesoc.unsw.edu.au/blob/dev/backend/server/events/events.go
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import (
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"encoding/json"
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"errors"
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"fmt"
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"io/ioutil"
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"net/http"
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"os"
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"time"
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)
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type (
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FbResponse struct {
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Data []FbRespEvent `json:"data"`
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Error FbRespError `json:"error"`
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}
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// FbRespEvent - struct to unmarshal event specifics
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FbRespEvent struct {
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Description string `json:"description"`
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Name string `json:"name"`
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Start string `json:"start_time"`
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End string `json:"end_time"`
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ID string `json:"id"`
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Place FbRespPlace `json:"place"`
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IsCancelled bool `json:"is_cancelled"`
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IsOnline bool `json:"is_online"`
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Cover FbRespCover `json:"cover"`
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}
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// FbRespError - struct to unmarshal any error response
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FbRespError struct {
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ErrorType int `json:"type"`
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Message string `json:"message"`
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}
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// FbRespCover - struct to unmarshal the URI of the cover image
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FbRespCover struct {
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CoverURI string `json:"source"`
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}
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// FbRespPlace - event location can come with added information, so we only take the name
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FbRespPlace struct {
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Name string `json:"name"`
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}
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// MarshalledEvents - struct to pack up events with the last update time to be marshalled.
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MarshalledEvents struct {
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LastUpdate int64 `json:"updated"`
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Events []Event `json:"events"`
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}
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// Event - struct to store an individual event with all the info we want
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Event struct {
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Name string `json:"name"`
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Description string `json:"description"`
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Start string `json:"start_time"`
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End string `json:"end_time"`
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ID string `json:"fb_event_id"`
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Place string `json:"place"`
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CoverURL string `json:"fb_cover_img"`
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}
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)
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const (
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FBEventPath = "/csesoc/events"
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FBGraphAPIPath = "https://graph.facebook.com/v15.0"
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)
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var FBAccessToken = os.Getenv("FB_TOKEN")
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// fetchEvents just fetches directly from the FB api, much of this code is bonked from the old website
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// i wish go had monads :(
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func queryFbApi() (FbResponse, error) {
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eventsReq, err := http.Get(
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fmt.Sprintf(
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"%s%s?access_token=%s&since=%d",
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FBGraphAPIPath, FBEventPath, FBAccessToken, time.Now().Unix(),
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),
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)
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if err != nil {
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return FbResponse{}, fmt.Errorf("there was an error fetching events from FB: %w", err)
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}
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defer eventsReq.Body.Close()
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if eventsReq.StatusCode != http.StatusOK {
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return FbResponse{}, fmt.Errorf("facebook api returned a http status: %d", eventsReq.StatusCode)
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}
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unparsedEvents, readErr := ioutil.ReadAll(eventsReq.Body)
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if readErr != nil {
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return FbResponse{}, fmt.Errorf("failed to read response body %w", readErr)
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}
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var apiResp FbResponse
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json.Unmarshal(unparsedEvents, &apiResp)
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if (apiResp.Error != FbRespError{}) {
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return FbResponse{}, errors.New("facebook api returned an error")
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}
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return apiResp, nil
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}
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// parseFbResponse takes a raw fb API response and converts it into a sequence of events
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func parseFbResponse(resp FbResponse) []Event {
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events := []Event{}
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for _, event := range resp.Data {
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if !event.IsCancelled {
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parsedEvent := Event{
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Name: event.Name, Description: event.Description,
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Start: event.Start, End: event.End,
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ID: event.ID, Place: event.Place.Name,
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CoverURL: event.Cover.CoverURI,
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}
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if event.IsOnline {
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parsedEvent.Place = "Online"
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}
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events = append(events, parsedEvent)
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}
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}
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return events
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}
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// GetFBEvents returns a sequence of all CSESoc events
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func GetFBEvents() ([]Event, error) {
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if rawFbData, err := queryFbApi(); err != nil {
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return parseFbResponse(rawFbData), nil
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} else {
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return nil, err
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}
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}

‎services/events-service/go.mod‎

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module events-service
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go 1.18

‎services/events-service/main.go‎

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package main
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import (
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"fmt"
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"log"
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"net/http"
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"sync"
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"time"
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)
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func main() {
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eventService := EventsService{
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cache: responseCache{
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cacheLock: sync.RWMutex{},
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expiryTime: time.Now(),
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},
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}
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http.HandleFunc("/events-api/v1/get-all", func(w http.ResponseWriter, r *http.Request) {
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fmt.Fprint(w, string(eventService.FetchEvents()))
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})
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// listen to port
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err := http.ListenAndServe(":8080", nil)
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if err != nil {
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log.Fatal(
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fmt.Errorf("failed to start server: %w", err))
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}
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}

‎services/events-service/response_cache.go‎

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package main
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import (
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"errors"
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"sync"
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"time"
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)
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// general cache configuration
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const (
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entryLifespan = 10 * time.Minute
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)
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// responseCache is maintains a cache of the responses returned by the FB API
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// entries are invalidated from the cache once the expiryTime has passed
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type responseCache struct {
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cachedResp []byte
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cacheLock sync.RWMutex
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expiryTime time.Time
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}
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// HasExpired determines if the entry in the response cache is still valid
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func (rc *responseCache) HasExpired() bool {
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rc.cacheLock.RLock()
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defer rc.cacheLock.RUnlock()
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return rc.expiryTime.Before(time.Now())
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}
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// Read reads the value of the response cache and returns an error if the entry has expired
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func (rc *responseCache) Read() ([]byte, error) {
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rc.cacheLock.RLock()
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defer rc.cacheLock.RUnlock()
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if rc.HasExpired() {
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return nil, errors.New("entry in the cached has expired")
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}
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return rc.cachedResp, nil
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}
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// RenewWith refreshes the contents of the cache with the new value and sets the expiry time
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// the expiry time is computed time.Now() + entryLifeSpan
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func (rc *responseCache) RenewWith(newResponse []byte) {
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rc.cacheLock.Lock()
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defer rc.cacheLock.Unlock()
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rc.cachedResp = newResponse
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rc.expiryTime = time.Now().Add(entryLifespan)
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}
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// Renew completely refreshes the expiry time for the value in the cache
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func (rc *responseCache) Renew() {
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rc.cacheLock.Lock()
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defer rc.cacheLock.Unlock()
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rc.expiryTime = time.Now().Add(entryLifespan)
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}

‎services/events-service/service.go‎

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package main
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import (
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"encoding/json"
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"fmt"
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"log"
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)
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type EventsService struct {
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cache responseCache
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}
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// FetchEvents reads from the FB API and process the data into a nice clean format
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// to be returned to the user :)
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func (service *EventsService) FetchEvents() []byte {
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if service.cache.HasExpired() {
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events, err := GetFBEvents()
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// if theres an error log it and renew the cache
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// keeping the old stale data
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if err != nil {
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log.Printf("Cache refresh service failed, error: %v", err)
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service.cache.Renew()
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} else {
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parsedJson, _ := json.Marshal(events)
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service.cache.RenewWith(parsedJson)
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}
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}
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resp, err := service.cache.Read()
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if err != nil {
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panic(fmt.Errorf(
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"fatal error, cache should not be empty at this point: %w", err,
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))
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}
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return resp
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}

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