by Wen Chuan Lee
9 min read

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I have a backlog of posts I’ve been deferring for the past year. Unfortunately with working at Amazon Payments I have not had the time I wanted to keep a steady stream of knowledge-sharing. I’ll do better!

Today we’ll talk about builders. Very often in microservices where we adhere to the contracts and boundaries of each service, we have to make service calls in the form of RPC or REST. That is, with each service’s separation of concerns, to perform a task, we have to make an API call to the downstream service to get a unit of work done, or to perform a callback upstream to notify the calling service that we have completed a task.

Build all the things!

What I want in a builder:

  1. Custom Logic for building the object
  2. Less boilerplate (and DRY)
  3. Abstraction of the construction of the data object from other data objects
  4. Extensibility that doesn’t leak too into the calling code (open-closed principle)
  5. Minimal parameter passing by the calling code to build the object.
  6. Force some fields to only be set by the builder (i.e calling code cannot explicitly set this value)
  7. Fluency. That is no explicit ‘get’ and ‘set’ in the method prefix.

It goes without surprise that at Amazon we have a lot of microservices powering the entire ecosystem of products we have at hand. Microservices have allowed teams to be able to grow outwards at scale and speed up development (when done right) as opposed to a monolithic service/codebase.1

In a perfect world, perhaps each microservice would have clearly defined contracts and a unified data-model that is shared across each service would make the calling code clean and simple. Let’s use an example, perhaps after our service completes a certain unit of work we need to call a downstream service to notify that we are done with processing. Downstream services may vend a builder to create a POST call with the required parameters for this.

We typically have to perform some form of message passing in the intended format to the downstream microservice. For example, you might have a service whose responsibility is to keep a record of payment processing, or for accounting.

Lombok to the rescue

Lombok would be the tool of choice for this as it removes a lot of the verbosity of Java. Particularly Lombok Builders. Suppose that we are provided/vended this builder, or that developers have written one for the ease of calling the downstream service.

import lombok.Builder;
import lombok.NonNull;

@Builder
public class DownstreamServiceCall {
    @Builder.default private long creationDate = System.currentTimeMillis();
    @NonNull private String uuid; 
    @NonNull private String customerId; 
    @NonNull private DataObject dataObject; 
}

This example would also use Defaults, which would automatically populate the field of the user does not specify a creationDate.

The calling code would then be able to do something like this, assuming the class has some transaction (data object) to get these values.

DownstreamServiceCall request = DownstreamServiceCall.builder()
                                    .uuid(transaction.getUUID())
                                    .customerId(transaction.getCustomerID())
                                    .dataObject(transaction.getDataObject())
                                    .build();

// Yes, this is a little weird, but I've seen service clients that require passing in ID/Headers as a separate paramter.
webClient.post(request.getUUID(), request);

I’m really a big fan of Builder pattern also because they do not require the user to figure out how to instantiate the object they need, and also abstracts the actual building of the request. Not having to call the setters on everything under the sun is kind of nice. Also, immutability is always sexy. Lombok really is a god send, never realized it when I first discovered it in my college days.

The Problem

However, the real world is never that simple. With enterprise services, very often these services will have a different data model or a different set of parameters required to be passed over. While microservices allow speed of development by independent teams and services are free to model their data according to their domain, what I’ve seen is that for legacy reasons, or in the interest of backwards-compatibility or to bridge the differences between each data model, there’s always some extra code required to massage the data before the request is sent to the service.

Since Lombok does not support custom builder logic, developers end up doing is something similar to this, as a ‘one-off’.

Note: Consider that Transaction is actually a legacy data object, and (for lack of better name), TransactionV2 is the improved data model.

//Transform the transaction object to get the fields required
Transaction legacyTransaction = convertToLegacyTransaction(transactionV2);

//UUID patch for backwards compatibility
String patchedUUID = UUIDUtility.patchUUID(legacyTransaction.getUUID());

DownstreamServiceCall request = DownstreamServiceCall.builder()
                                .uuid(patchedUUID)
                                .customerId(legacyTransaction.getCustomerID())
                                .dataObject(legacyTransaction.getDataObject())
                                .build();

webClient.post(patchedUUID, request);

Developers end up making a private function like convertToLegacyTransaction, or move their code to some Utility class, or if they’re feeling fancy, a Factory class to handle the transmutation of this data object. Often times it is for the sole purpose of the DownstreamService and nothing else.

This is alright until other upstream microservices also attempt to call this downstream service. What I’ve seen happen is that developers end up duplicating code to avoid having a dependency on the package containing this Utility or Factory class, or they have to pull in an entire depenedency just to use this factory class. Pretty soon the boundaries between each library/package is blurred, and we have all sorts of services using this code to make a call to the downstream service. Then, when there’s a new business requirement or the domain has shifted, there’s going to be a lot of pain updating these calls to pass down a new flag or value.

While Lombok is great for what it’s designed to do, sometimes there’s a need to have a custom Builder to abstract these changes away from the calling code. The downstream service can vend a specialized builder to handle TransactionV2 data Objects, or to vend it as a separate library for users.

The Solution

Hence, to make your own custom builder, you might think you’ll not be able to use Lombok and you’ll have to write a lot of boilerplate code, dying a little inside as you favor less code so you have less to maintain. A solution is discussed in the Lombok documentation specified and highlighted in this stackOverflow question, which is to define a skeleton for yourself.

The goal I wanted when refactoring code like this was to have a user just do something like this:

    DownstreamServiceCall request = DownstreamServiceCallBuilder.builder()
                                    .transactionV2(transactionV2).build();
    webClient.post(request.getUUID(), request);

I also want to leverage Lombok whenever possible.

Here’s what I got after trying out various forms:

import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.NonNull;
import lombok.Setter;
import lombok.experimental.Accessors;


@Accessors(fluent = true)
@AllArgsConstructor(access = AccessLevel.PRIVATE)
public class DownstreamServiceCall {
    @Getter @NonNull private final String uuid;
    @Getter @NonNull private final long creationDate;
    @Getter @NonNull private final String customerId;
    @Getter @NonNull private final DataObject dataObject;

    private static final String DATE_MILLIS_PAD_FORMAT = "%014d";
    private static final String UUID_FORMAT = "%s.%d";
    
    @NoArgsConstructor(staticName = "builder")
    @Accessors(chain=true, fluent=true)
    public static final class Builder {
        //Only allow calling code to set some parameters. 
        @Setter @NonNull private TransactionV2 transactionV2;
        private String uuid;
        private long creationDate;
        
        public DownstreamServiceCall build() {
            this.uuid = patchUUID(transactionV2.getUUID());
            
            if (transactionV2.getCreationDate() == null) {
                this.creationDate = String.format(DATE_MILLIS_PAD_FORMAT, System.currentTimeMillis());
            }
            Transaction legacyTransaction = convertToLegacyTransaction(transactionV2);
            
            return new DownstreamServiceCall(uuid, creationDate, legacyTransaction.getCustomerID(), legacyTransaction.getDataObject());
        }
        
        private String patchUUID(String originalUUID) {
            //do any legacy/backwards compatible logic, even pass in any parameters you might need!
            String modifiedUUID = String.format(UUID_FORMAT, originalUUID, computeVersionNumber());
            return modifiedUUID;
        }
        
        private Transaction convertToLegacyTransaction(TransactionV2 transactionV2) {
            Transaction legacyTransaction;
            //do any transformation logic here
            return legacyTransaction;
        }
    }
}

With this, calling code will now be able to do just this:

    DownstreamServiceCall request = DownstreamServiceCall.builder()
                                    .transactionV2(transactionV2)
                                    .build();
    webClient.post(request.uuid(), request);

What did we achieve?

Essentially we made a wrapper class DownstreamServiceCall , and inside it a static Builder class that will:

  • Handle the custom logic to construct an object
  • Encapsulate any form of data massaging and kept validation inside the builder
  • Allow for changes to this builder class to be hidden away from the calling code as long as you have the data required
  • One single point of extension if the interface/contract of the DownstreamService changes
  • Force calling code to use a builder, as the access level for the wrapper class is set to private
  • Enforce that once a DownstreamServiceCall is instantiated, the values it carries will never change and can only be get not set. Yay #Immutability!
  • Fluent builder, and Bob’s your uncle!2

This ensures transformation logic is kept in sync and all calling code will not have duplicate logic of how to construct such an object. In the future, if we have transactionV3 or another data source, we can just extend another builder for it. By setting this pattern, we ensure future attempts to hack around this pattern are subjugated.

The initial versions I had forced me to do weird things like instantiate the inner class with new DownstreamServiceCall.Builder in the calling code, then set the fields, before build(), as I did not use the @NoArgsConstructor(staticName) annotation. The annotation reduces the need for this boilerplate:

/**
 * @return an instance of the builder class.
*/
public static Builder builder() {
    return new Builder();
}

Of course, the example looks pretty simple and there may be other methods that accomplish the same goal, but for the internal use case I had, I could extend this builder to do anything it needed to create a well formed call and bring convenience and immutability to the calling code. Regarding the extra lines of code, it really wasn’t that much more when I merged all the data massaging logic into one class.

Special thanks for Wouter Sieling for his guidance on this builder method. I thought I’d share it given I haven’t seen an implementation that would fit my use-case when I was doing this a while back. Mainly I wanted fluency and to avoid developers from adding more custom logic outside of this builder class to make a service call.

Thanks for reading!

  1. There’s a lot of good books I’ve been recommended on this, but I have not read any of them to recommend any. AWS does have a cliffnotes of why they’re the de-facto here though! 

  2. PS: For those that did not get the joke, “Bob’s your uncle” because of the popular kid tv series, Bob the Builder. Get it?