How to run 2 queries sequentially in a Android RxJava Observable?

The operator to do that would be merge, see http://reactivex.io/documentation/operators/merge.html.

My approach would be to create two observables, let's say observableLocal and observableRemote, and merge the output:

Observable<Object> observableLocal = Observable.create(...)Observable<Object> observableRemote = Observable.create(...)Observable.merge(observableLocal, observableRemote)          .subscribe(subscriber)

If you want to make sure that remote is run after local, you can use concat.

Lukas Batteau's answer is best if the queries are not dependent on one another. However, if it is necessary for you obtain the data from the local SQLite query before you run the remote query (for example you need the data for the remote query params or headers) then you can start with the local observable and then flatmap it to combine the two observables after you obtain the data from the local query:

   Observable<Object> localObservable = Observable.create(...)   localObservable.flatMap(object ->    {       return Observable.zip(Observable.just(object), *create remote observable here*,            (localObservable, remoteObservable) ->            {               *combining function*           });   }).subscribe(subscriber);

The flatmap function allows you to transform the local observable into a combination of the local & remote observables via the zip function. And to reiterate, the advantage here is that the two observables are sequential, and the zip function will only run after both dependent observables run.

Furthermore, the zip function will allow you to combine observables even if the underlying objects have different types. In that case, you provide a combining function as the 3rd parameter. If the underlying data is the same type, replace the zip function with a merge.

You can try my solutions, there are several ways to resolve your problem.
To make sure it's working, I created a stand alone working example and use this API to test: https://jsonplaceholder.typicode.com/posts/1

private final Retrofit retrofit = new Retrofit.Builder()            .baseUrl("https://jsonplaceholder.typicode.com/posts/")            .addConverterFactory(GsonConverterFactory.create())            .addCallAdapterFactory(RxJavaCallAdapterFactory.create())            .build();    private final RestPostsService restPostsService = retrofit.create(RestPostsService.class);    private Observable<Posts> getPostById(int id) {        return restPostsService.getPostsById(id);    }

RestPostService.java

package app.com.rxretrofit;import retrofit2.http.GET;import retrofit2.http.Path;import rx.Observable;/** * -> Created by Think-Twice-Code-Once on 11/26/2017. */public interface RestPostsService {    @GET("{id}")    Observable<Posts> getPostsById(@Path("id") int id);}

Solution1: Use when call multiple tasks in sequences, the result of previous tasks is always the input of the next task

getPostById(1)                .concatMap(posts1 -> {                    //get post 1 success                    return getPostById(posts1.getId() + 1);                })                .concatMap(posts2 -> {                    //get post 2 success                    return getPostById(posts2.getId() + 1);                })                .concatMap(posts3 -> {                    //get post 3success                    return getPostById(posts3.getId() + 1);                })                .subscribeOn(Schedulers.io())                .observeOn(AndroidSchedulers.mainThread())                .subscribe(finalPosts -> {                    //get post 4 success                    Toast.makeText(this, "Final result: " + finalPosts.getId() + " - " + finalPosts.getTitle(),                            Toast.LENGTH_LONG).show();                });

Solution2: Use when call multiple tasks in sequences, all results of previous tasks is the input of the final task (for example: after uploading avatar image and cover image, call api to create new user with these image URLs):

Observable                .zip(getPostById(1), getPostById(2), getPostById(3), (posts1, posts2, posts3) -> {                    //this method defines how to zip all separate results into one                    return posts1.getId() + posts2.getId() + posts3.getId();                })                .flatMap(finalPostId -> {                    //after get all first three posts, get the final posts,                    // the final posts-id is sum of these posts-id                    return getPostById(finalPostId);                })                .subscribeOn(Schedulers.io())                .observeOn(AndroidSchedulers.mainThread())                .subscribe(finalPosts -> {                    Toast.makeText(this, "Final posts: " + finalPosts.getId() + " - " + finalPosts.getTitle(),                            Toast.LENGTH_SHORT).show();                });

AndroidManifest

 <uses-permission android:name="android.permission.INTERNET"/>

root build.gradle

// Top-level build file where you can add configuration options common to all sub-projects/modules.buildscript {    repositories {        jcenter()    }    dependencies {        classpath 'com.android.tools.build:gradle:2.3.3'        classpath 'me.tatarka:gradle-retrolambda:3.2.0'        classpath 'me.tatarka.retrolambda.projectlombok:lombok.ast:0.2.3.a2'        // NOTE: Do not place your application dependencies here; they belong        // in the individual module build.gradle files    }    // Exclude the version that the android plugin depends on.    configurations.classpath.exclude group: 'com.android.tools.external.lombok'}allprojects {    repositories {        jcenter()    }}task clean(type: Delete) {    delete rootProject.buildDir}

app/build.gradle

apply plugin: 'me.tatarka.retrolambda'apply plugin: 'com.android.application'android {    compileSdkVersion 26    buildToolsVersion "26.0.1"    defaultConfig {        applicationId "app.com.rxretrofit"        minSdkVersion 15        targetSdkVersion 26        versionCode 1        versionName "1.0"        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"    }    buildTypes {        release {            minifyEnabled false            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'        }    }    compileOptions {        sourceCompatibility JavaVersion.VERSION_1_8        targetCompatibility JavaVersion.VERSION_1_8    }}dependencies {    compile fileTree(dir: 'libs', include: ['*.jar'])    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {        exclude group: 'com.android.support', module: 'support-annotations'    })    compile 'com.android.support:appcompat-v7:26.+'    compile 'com.android.support.constraint:constraint-layout:1.0.2'    testCompile 'junit:junit:4.12'    provided 'org.projectlombok:lombok:1.16.6'    compile 'com.squareup.retrofit2:retrofit:2.3.0'    compile 'com.squareup.retrofit2:converter-gson:2.3.0'    compile 'com.squareup.retrofit2:adapter-rxjava:2.3.0'    compile 'io.reactivex:rxandroid:1.2.1'}

model

package app.com.rxretrofit;import com.google.gson.annotations.SerializedName;/** * -> Created by Think-Twice-Code-Once on 11/26/2017. */public class Posts {    @SerializedName("userId")    private int userId;    @SerializedName("id")    private int id;    @SerializedName("title")    private String title;    @SerializedName("body")    private String body;    public int getUserId() {        return userId;    }    public void setUserId(int userId) {        this.userId = userId;    }    public int getId() {        return id;    }    public void setId(int id) {        this.id = id;    }    public String getTitle() {        return title;    }    public void setTitle(String title) {        this.title = title;    }    public String getBody() {        return body;    }    public void setBody(String body) {        this.body = body;    }}

By the way, use Rx + Retrofit + Dagger + MVP pattern is a great combine.