From Cart Pick to Put Walls

A critical part of any e-commerce company is getting product to its customers. While many of the customer experience discussions that you hear about companies focus on their website and apps or customer service and support, we often forget to think about those companies delivering their customers’ products when the company said they would. This part of the promise made (or implied) for customers is critical for building trust and providing a great end-to-end customer experience. Most large e-commerce companies operate — or pay someone else to operate — one or more “fulfillment centers”, which is where products are stored and combined with other items that need to be sent to the customer. zulily’s unique business model means we work with both big brands and boutique, smaller vendors with a variety of different capabilities, and so our products are inspected for quality, frequently bagged to keep clothing from getting dirty and often need barcoding (as many smaller vendors may not have them). The quality of zulily’s fulfillment processes drives our ability to deliver on our promises to customers and zulily’s software drives those fulfillment processes.

All fulfillment center systems start with a few basic needs: be able to receive products in from vendors, store products in a way that they can be later retrieved, and ship the product to customers. “Shipping product out,” also known as “outbound” is the most expensive operation inside the fulfillment center, so we have invested heavily in making it efficient. The problem seems simple at first glance. You gather product for customer shipment, put products in boxes, put labels on the boxes, and hand the box to UPS or USPS, etc.. The trick is making this process as efficient as possible. When zulily first started, each associate would walk the length of the warehouse picking each item and sorting it into 1 of 20 shoebox sized bins they had in their cart with each bin representing a customer shipment. Once all of the shipments had been picked, the picker delivers the completed cart to a packing station. The job of collecting products to be shipped out is known as “picking” and when our warehouse was fairly small, this strategy of one person picking the whole order worked fine. As the company has grown, our warehouses did too – some of our buildings have a million square feet of storage spread over multiple floors. Now these pickers were walking quite a long way in order for just 20 shipments. We could have just increased the size or quantity of the carts, but this is a solution that costs more as the company grows.  In addition, concerns about safety related to pulling more or larger carts and the complexities of taking one cart to multiple floors of a building make this idea impractical, to say the least.

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A pick cart. Each of the 20 slots on the cart represents a single customer shipment. The picker, guided by an app on a mobile device, walks the storage area until they’ve picked all of the items for the 20 shipments. We call this process “pick to shipment” because no further sorting is necessary to make sure each shipment is fully assembled.

We needed a solution that would allow pickers to spend less time walking between bins and more time picking items from those bins. We have developed a solution such that the picking software tries to keep a given picker within a zone of 10-20 storage aisles and invested in a conveyor system to carry the picked items out of the picking locations. The picker focuses on picking everything that can be picked within their zone and there’s no need for a picker to leave a zone unless they are needed in another zone. The biggest difference from the old model is that the picker is no longer assembling complete shipments. If you ordered a pair of shoes and a t-shirt from zulily, it’s unlikely that those two items would be found in the same zone due to storage considerations. Instead of an individual picker picking for 20 orders, we now have one picker picking for many orders at the same time, but staying within a certain physical area of the building. This is considerably more efficient for the pickers, but it means that we now needed a solution to assemble these zone picks into customer shipments.

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The picker picks for multiple shipments into a single container. Because the sorting into customer shipments happens later, this solution is called “pick to sort”.

In order to take the efficiently picked items and sort them into the right order to be sent to our customers, we have implemented a sorting solution that uses a physical solution we call a “put wall”. A put wall looks like a large shelf with no back divided into sections (called “slots”), each measuring about one foot cubed. Working at these put walls is an employee (called a “putter”) whose job is to take products from the pick totes and sort them into a slot in that put wall. Each slot in the wall is assigned to a shipment. Once all the products needed for a given shipment have been put into the slot, an indicator light on the other side of the wall lets a packer know that the shipment is ready to be placed into a box and shipped out to our customer. In larger warehouses, having just one put wall is not practical because putters would end up having to move too much distance and all the efficiency gained in packing would be lost on the putting side, so defining an appropriate size for each put wall is critical. This creates an interesting technical challenge as we have to make sure that the right products all end up in the put wall at the right time. Our picking system has to make sure that once we start picking a shipment to a wall that all the other products for that shipment also go to that wall as quickly as possible. This challenge is made more difficult by the physical capacity of the put walls. We need to limit how much is going to the wall to avoid a situation where there is no slot for a new shipment to go. We also have to make sure that each of the walls have enough work so we don’t have idle associates. When selecting shipments to be picked, we must include shipments that are due out today, but also include future work to make the operation efficient.  To do this, we have pickers rotate picking against different put walls to make sure that they get an even spread of work. A simple round-robin rotation would be naive, since throughput of the put walls is determined by humans with a wide range of different work rates. In order to solve this problem, we turn to control theory to help us select a put wall for a picker based on many of the above requirements. We also need to make sure that when the first product shows up for a shipment there is room in the wall for it.

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As totes full of picked items are conveyed to the put wall, a putter scans each item and puts them into a slot representing a customer shipment. He is guided by both his mobile device and flashing lights on the put wall which guide him to the correct slot.

As we scaled up our operation, we initially saw that adding more pickers and put walls was not providing as much gain in throughput as we expected. In analyzing the data from the system, we determined that one of the problems was how we were selecting our put walls. Our initial implementation would select a wall for a shipment based on that wall having enough capacity and need. The problem with this approach is that we didn’t consider the makeup of each of the shipments. If you imagine a shipment that is composed of multiple products spread throughout the warehouse, you have situations where a picker has to walk through their zone N times, where N is the number of put walls we are using at any given time. As we turn on more and more put walls, that picker will have to walk through the zone that many more times. We realized that if we can create some affinity between zones and walls, we can limit the amount of put walls that a picker needs to pick and make them more efficient. We did this by assigning put walls a set of zones and try to make the vast majority of shipments for that put wall come from those zones. While we need to sometimes have larger sets that normal to cover a given shipment, we can overall significantly improve pick performance and increase the overall throughput for putters and packers.

And that’s really just the beginning of the story for a small part of our fulfillment center software suite. As the business grows, we continue to find new ways to further optimize these processes to make better use of our employees’ time and save literally millions of dollars while also increasing our total capacity using the same buildings and people! This is true of most of the software in the fulfillment space – improved algorithms are not just a fun and challenging part of the job, but also critical to the long-term success of our business.
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The Thrill-Ride Ahead for zulily Engineers

zulily is an e-commerce company that is changing the retail landscape through our ‘browse and discover’ business model.  Long term, there are tremendous international expansion opportunities as we change the way people shop across the planet. At zulily we’re building a future where the simultaneous release of 9,000 product styles across 100+ events can occur seamlessly in multiple languages across multiple platforms. From an engineering perspective, as we expand globally the size and scope of our technical challenge is nothing short of localization’s Mount Everest climb.

Navigating steep localization challenges is not new to companies expanding globally yet zulily faces a particularly unique thrill-ride ahead. For me personally, this is incredibly exciting. Prior to coming to zulily one role I held at my former company was global readiness – to ensure the products and services that the company delivered to customers were culturally, politically and geographically appropriate. I was in the unique role of mitigating the company’s risk of negative press, boycotts, protests, lawsuits or being banned by governments. The content my team reviewed was never considered life-threatening until the 2005 incident when the cultural editor of Jyllands-Posten in Denmark commissions twelve cartoonists to draw cartoons of Islamic prophet Muhammad. Those cartoons were published and their publication led to the loss of life and property. Suddenly my team took notice. Having content thoughtfully considered and ready for global markets took on a grave seriousness moving from a ‘nice-to-have’ to a ‘must-have’ risk management function. While my role was to ensure the political correctness (neutrality) of content across 500+ product groups, the group I led rarely dealt with the size and scope of technical localization challenges that zulily is facing as we expand globally.

As companies expand globally it’s important for the employees to conceptually shift their mindset from a U.S. centric perspective to a global view. This paradigm leap in how the employees of an organization consider themselves is significant. When people are increasingly thinking globally about their role and the impact of their decisions on a global audience, specifically in the area of technology and how we enable our platforms, tools and systems to be ‘world-ready’, opportunities for growth and development naturally occur while cultural content risks reduce. Today all of the content on our eight country-specific sites is in English yet we are now thinking about how to tackle bigger challenges in the future, such as supporting multiple languages. The technical implementation for international expansion has enabled our developers and product managers to gain a new appreciation for the importance of global readiness and the challenge of ‘going international’.

zulily offers over 9,000 product styles through over 100 merchandising events on a typical day. As we expand into new markets around the globe we face an extraordinary challenge from both an engineering and operations perspective. Imagine, every day at 6 a.m. PT we’re publishing the content equivalent of a daily edition of The New York Times (about one half million new words per day or over 100 million new words per year).  Another way to conceptualize the technical problem – each day we offer roughly the same number of SKUs of what you would find in a typical Costco store. Launching a new Costco store every day is difficult enough in one language yet as we scale our offerings globally the complexity to simultaneously produce this extremely high volume of content in multiple languages grows exponentially. Arguably, no e-commerce company in the world is publishing the volume of text that zulily produces on a daily basis. Further, the technical challenge is amplified because the massive volume of content must be optimized to work across multiple platforms, e.g., iPhone, iPad, Android and web based devices. In fact, over 56% of our orders are placed on mobile devices. From a user-experience across platforms and languages, text expansion and contraction become a significant issue. European languages such as French, German and Italian may require up to 30% more space than English. Double byte characters such as Chinese and Japanese will require less space.

The content on our sites is produced in-house each day by our own talented copy writers and editors. Not unlike publishing a newspaper the deadline for a 6 a.m. launch of fresh, new product styles is typically the night before. Last minute edits can happen as late as midnight! While this makes for an exciting and dynamic environment, it requires some of the brightest engineering and operational minds on the planet to bring it all together with the quality and performance we expect.

From a technical perspective our recent global expansion to Mexico, Hong Kong and Singapore faced typical localization hurdles. We needed to implement standard solutions to accommodate additional address line fields in the shipping address; ensure proper currency symbols are displayed; coding rules that allowed us to process orders without postal codes which are not required in Hong Kong.

Address Field Requirement Example:

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Bold = unique as compared to U.S. requirement

As we continue to move into new markets we’ll be driven to apply new solutions to traditional localization problems simply based upon the sheer volume of content and arduous daily production requirements. These two forces alone combine to drive creativity, invention and technological breakthroughs that will accelerate our growth and expansion. Our team at zulily is now exploring various strategies in engineering and operations to tackle the local/regional cultural differences across markets while also bringing zulily to our customers in their own language. We continue to hire the brightest minds to help us invent solutions for a new way of shopping. At zulily, we tell our customers ‘something fresh every day’. Our engineers enable that reality by creating something fresh every day. Our ambition to grow globally will give everyone at zulily that opportunity.

Follow us on Twitter: @zulilytech | @jmstutz