Reprinted from: http://ec.sina.com.cn/ec/2011-05-23/12599.html
Although e-commerce is an emerging industry, it is essentially a retail industry. Aside from various electronic and network factors, it is still a basic model of buying at a low price and selling at a high price. However, due to the new characteristics of electronic, network and remote, e-commerce is different from traditional retail.
Except for hotel and travel service providers represented by Ctrip and eLong, and service information group purchasers represented by Lashou and Meituan, all e-commerce companies are facing the need for physical inventory management. Among them, the back-end procurement and inventory control, as well as the corresponding inventory, logistics distribution and after-sales service, are very professional and complex, and the experience of the traditional retail industry may not be suitable for the management of e-commerce retail.
Inventory, literally understood, refers to the stock in the warehouse. For production enterprises, inventory can be divided into two categories: production raw materials and production results. For retail enterprises, there is no production process, all goods are purchased from upstream, stored in warehouses, and then sold to downstream. Inventory, which is the sum of the items in the warehouse.
But the inventory in the warehouse cannot be generalized. Even a SKU will belong to different states. To give a simple example, in the process of physical circulation, bumps and damages will inevitably occur, and these damaged products cannot be sold normally, and must be distinguished in system management. In e-commerce operations, since the purchase and delivery are asynchronous in time and space, there is also a time gap between the customer placing the order and the delivery, so the inventory structure must be distinguished.
In order to adapt to this asynchronous sales process, the inventory of e-commerce enterprises is generally divided into the following parts in the system structure:
A. Saleable Inventory (S)
Sellable inventory is the inventory displayed on the front desk of the website, and it is also the largest component of inventory. In most e-commerce enterprises, the front-end website will keep data synchronization with the back-end WMS (Warehouse Management System) and make judgments. When "Saleable Inventory > 0", the product is available for purchase, and the front-end website will display that the product is available for sale; and once "Sellable Inventory < 0", the front-end website will show that the product is out of stock. Generally speaking, out of stock does not mean that there is no stock in the warehouse, but just that there is no saleable stock.
Most companies will only show if there is inventory in the front desk, but this can actually be done in more detail.
For example, on the front desk of JD.com, when customers select different receiving areas, the system will display whether there is stock in stock according to the inventory data of each sub-warehouse to help customers purchase and achieve a better customer experience (Figure 1).
In the excellent Amazon system, when the number of available inventory is very small, it will prompt customers at the front desk that the number of customers is small, and ask customers to step up their purchases to increase the conversion rate (Figure 2).
When the customer purchases the product and confirms the order, the front-end website will first send a request to the back-end system to check the quantity of the ordered product and the current saleable stock quantity. If the number of saleable inventory > the number of products ordered, the front-end website will be notified of success, otherwise, the front-end will be notified that the inventory is insufficient and the customer will be reminded.
After a new order is generated, the inventory purchased by the customer will be reserved for subsequent delivery. The amount of available inventory in the system is reduced, and where does the reduced available inventory go? This becomes the next part.
B. Order Occupies Inventory (O)
When an order is generated, the quantity of available inventory decreases, the quantity of Order Occupied Inventory increases, and the changed quantity is the quantity of products in the order, which is easy to understand.
The reason for setting up an order to occupy inventory is that the generation of the order and the delivery of the warehouse are asynchronous in time. The advantages of this are: to ensure that the inventory that has already generated the order is guaranteed, and these customers can receive the goods smoothly; and when the customer places an order, the product can be guaranteed to be delivered. If the order is not established to occupy the inventory, there will be an embarrassing situation in which the inventory is found to be out of stock after the customer places the order.
When processing an order, it is only for the inventory that has been occupied by the order, and has nothing to do with the sales at the front desk. After the order is released from the warehouse, only the inventory occupied by the order is deducted in the system.
C. Unsellable Inventory (U)
It is often said that theory should be consistent with practice. This sentence is applied to inventory management, that is, the system records of inventory need to correspond to the physical inventory.
In the previous example, the product cannot be sold due to damage and must also have a corresponding status in the system. In actual operation, there are many reasons why normal sales cannot be made, such as damaged packaging, performance failure, wrong model labeling and so on. In order to keep the theory in line with reality, this part of the inventory is also defined as Unsellable Inventory in the system.
不可销售库存在系统中的标注方法有两类。一类是使用不同的SKU代号来区别，例如某一正常商品的SKU编码是351038，它所对应的不可销售库存的SKU编码则是351038U；另外一种方式是使用同一种SKU，但是专门开辟一个不可销售库存区，所有不可销售的库存统一管理。
D. 锁定库存(L)
在销售中，经常会使用的一种促销方式是降价，这一方式的效果会非常好，成功的降价促销可以在很短时间内将商品一售而空，可销售库存直接转化为订单占用库存。
但是有一些情况下，销售方并不希望这么快就将所有的库存都售出。有的时候是因为所有库存全部作降价促销的成本很高，有的时候是防止竞争对手的恶意采购，更多的情况下，则是希望将这一产品的降价作为引子，带动网站的流量和整体销售，这就需要将促销分批次进行。
为达到以上的目的，会采用锁定库存(Locked Inventory)的方式。库存被锁定后，无法直接销售。促销进行一段时间后，可用库存为0，无法继续销售，必须在解除锁定后才能转化为可销售库存，继续进行销售。
E. 虚库存(V)
以上所说的，都是指在实物在库房中的库存。但库房的总容积量是一定的，不可能无限制的扩展。而依据长尾理论，电子商务的最大优势则是几乎无限的商品展示和销售能力。如何将有限的库房处理能力和无限的可销售商品联系赶来呢？方法早就有了，虚库存(Virtual Inventory)。
有一些产品，虽然库房中并没有，或者并没有很多，但是供应渠道非常通畅，可以在很短的时间内送到库房中，变为库存；另外一些产品，销售量少，库存的管理难度大，只有当产生订单后，才向供应商采购。这部分不在实际的库存中，但是可以很快采购到的货品就叫做虚库存。
虚库存的存在，是为了使前台网站的可销售数量大于实际可销售数量。当存在虚库存时，电商库存公式会变成：
S=I-O-U-L+V
F. 调拨占用库存(T)
很多B2C企业有着超过一个以上的库房。多个库房的设置，主要是因为规模发展到一定程度后，库存量很大，很难在一个单独的库房中存储，另外，也经常会在客户聚集地附近设立库房，以满足当地客户的需求。
各个库房之间，必然存在着库存的分派和调拨。当产生调拨计划后，调出地库房的某一部分库存就会被占用，这部分库存被称为调拨占用库存(Transport Inventory)。调拨占用库存和订单占用库存的性质相似。当存在调拨占用库存后，电商库存公式变成：
S=I-O-U-L+V-T
G. 调拨中库存(A)
库存的调拨，必然会存在一段时间，库存既不存在于调拨出库房，也不存在于调拨入库房，这一部分库存就像漂在空中一样，称为调拨中库存(Air Inventory)。设1号库房为调拨出库房，2号库房为调拨出库房，在调拨发货前，这两个库房的库存结构为：
I1=S1+O1+U1+L1-V1+T1
I2=S2+O2+U2+L2-V2+T2
I=S+O+U+L-V+T
若从1号库房调拨出量为A的库存到B库房，在1号库房调拨发出后，2号库房收到调拨前，两库房的库存结构为：
I1=S1+O1+U1+L1-V1+T1-A
I2=S2+O2+U2+L2-V2+T2
I=S+O+U+L-V+T-A
可以看到，两个库房的总库存减少了，调拨中库存在路上，只能计在财务库存中，而并不能计入实物库存。只有当调拨完成后，库存进入2号库房，总库存才会恢复。
I1=S1+O1+U1+L1-V1+T1-A
I2=S2+O2+U2+L2-V2+T2+A
I=S+O+U+L-V+T
以上是通用性的一些库存结构综述，在各家公司不同的商业模式和运营流程中，有着各自的特殊性，会使用到更加复杂的库存结构方式，以适应本公司的实际情况，这里不作太深入的探讨。